* SAMV7: SPI: SPI-Freq. 40MHz; VARSELECT; hw-features

This change adds the following improvements:
  - Increase the allowed SPI-Frequency from 20 to 40 MHz.
  - Correct and rename the "VARSELECT" option This option was included in the code as "CONFIG_SPI_VARSELECT" but nowhere  defined in a Kconfig file. The patch renames it to "CONFIG_SAMV7_SPI_VARSELECT" and corrects the implementation according the datasheet of Atmel. In short, this option switches the processor from "fixed peripheral selection" (single device) to "variable peripheral selection" (multiple devices on the bus).
  - Add a new Function to the interface to control the timing and delays of the chip according the ChipSelect lines. This function can control the delay between the assertion of the ChipSelect and the first bit, between the last bit and the de-assertion of the ChipSelect and between two ChipSelects. This is needed to tune the transfer according the specification of the connected devices.
  - Add three "hw-features" for the SAMV7, which controls the behavior of the ChipSelect:
    1. force CS inactive after transfer: this forces a (short) de-assertion of the CS after a transfer, even if more data is
      available in time
    2. force CS active after transfer: this forces the CS to stay active after a transfer, even if the chip runs out of data. Btw.: this is a prerequisit to make the LASTXFER bit working at all.
    - escape LASTXFER: this suppresses the LASTXFER bit at the end of the  next transfer. The "escape"-Flag is reset automatically.

include/nuttx/spi/spi.h

@@ -127,6 +127,27 @@
 
 #define SPI_SETFREQUENCY(d,f) ((d)->ops->setfrequency(d,f))
 
+/****************************************************************************
+ * Name: SPI_SETDELAY
+ *
+ * Description:
+ *   Set the SPI Delays in nanoseconds. Optional.
+ *
+ * Input Parameters:
+ *   dev        - Device-specific state data
+ *   startdelay - The delay between CS active and first CLK
+ *   stopdelay  - The delay between last CLK and CS inactive
+ *   csdelay    - The delay between CS inactive and CS active again
+ *
+ * Returned Value:
+ *   Returns the actual frequency selected
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_SPI_CS_DELAY_CONTROL
+#  define SPI_SETDELAY(d,a,b,c) ((d)->ops->setdelay(d,a,b,c))
+#endif
+
 /****************************************************************************
  * Name: SPI_SETMODE
  *
@@ -189,15 +210,35 @@
 #  define SPI_HWFEATURES(d,f) \
   (((d)->ops->hwfeatures) ? (d)->ops->hwfeatures(d,f) : ((f) == 0 ? OK : -ENOSYS))
 
-  /* These are currently defined feature flags */
+  /* These are currently defined feature flags:
+   *
+   *   Bit 0: HWFEAT_CRCGENERATION
+   *          Hardware CRC generation
+   *   Bit 1: HWFEAT_FORCE_CS_INACTIVE_AFTER_TRANSFER
+   *          CS rises after every Transmission, also if we provide new data
+   *          immediately
+   *   Bit 2: HWFEAT_FORCE_CS_ACTIVE_AFTER_TRANSFER
+   *          CS does not rise automatically after a transmission, also if
+   *          the spi runs out of data (for a long time)
+   *   Bit 3: HWFEAT_ESCAPE_LASTXFER
+   *          Do not set the LASTXFER-Bit at the last word of the next
+   *          exchange, Flag is auto-resetting after the next LASTXFER
+   *          condition. (see spi_exchange)
+   */
 
 #  ifdef CONFIG_SPI_CRCGENERATION
-#    HWFEAT_CRCGENERATION (1 << 0) /* Bit 0: Hardward CRC generation */
+#    define HWFEAT_CRCGENERATION                     (1 << 0)
+#  endif
+
+#  ifdef CONFIG_SPI_CS_CONTROL
+#    define HWFEAT_FORCE_CS_INACTIVE_AFTER_TRANSFER  (1 << 1)
+#    define HWFEAT_FORCE_CS_ACTIVE_AFTER_TRANSFER    (1 << 2)
+#    define HWFEAT_ESCAPE_LASTXFER                   (1 << 3)
 #  endif
 
 #else
   /* Any attempt to select hardware features with CONFIG_SPI_HWFEATURES
-   * deselected will cause an assertion.
+   * deselected will return an -ENOSYS error.
    */
 
 #  define SPI_HWFEATURES(d,f) (((f) == 0) ? OK : -ENOSYS)
@@ -431,6 +472,10 @@ struct spi_ops_s
   CODE void     (*select)(FAR struct spi_dev_s *dev, enum spi_dev_e devid,
                   bool selected);
   CODE uint32_t (*setfrequency)(FAR struct spi_dev_s *dev, uint32_t frequency);
+#ifdef CONFIG_SPI_CS_DELAY_CONTROL
+  CODE int      (*setdelay)(FAR struct spi_dev_s *dev, uint32_t a, uint32_t b,
+                  uint32_t c);
+#endif
   CODE void     (*setmode)(FAR struct spi_dev_s *dev, enum spi_mode_e mode);
   CODE void     (*setbits)(FAR struct spi_dev_s *dev, int nbits);
 #ifdef CONFIG_SPI_HWFEATURES