mpfs: ddr: add DDR training

This adds DDR training. The training has a small chance of failing,
and then the training is restarted.

DDR training cannot be done meaningfully while the software is
in DDR. If the system is intended to run from eNVM, like a
bootloader, the linker script should be tuned to utilize the envm
region as follows:

  envm (rx)   : ORIGIN = 0x20220100, LENGTH = 128K - 256
  l2lim (rwx) : ORIGIN = 0x08000000, LENGTH = 1024k

256 bytes are reserved for the system; The fixed block may be
installed from the 'hart-software-services' -repository:
https://github.com/polarfire-soc/hart-software-services.git

For example, the 256-byte image: hss-envm-wrapper-bm1-dummySbic.bin
may be prepended on the nuttx bootloader image in the following
manner:

 cat hss-envm-wrapper-bm1-dummySbic.bin > nuttx_bootloader.bin
 cat nuttx.bin >> nuttx_bootloader.bin
 riscv64-unknown-elf-objcopy -I binary -O ihex --change-section-lma
  *+0x20220000 nuttx_bootloader.bin flashable_image.hex

This provides an image 'flashable_image.hex' that may be flashed on
the eNVM region via Microsemi Libero tool.

Signed-off-by: Eero Nurkkala <eero.nurkkala@offcode.fi>

arch/risc-v/src/mpfs/mpfs_ddr.h

@@ -0,0 +1,47 @@
+/****************************************************************************
+ * arch/risc-v/src/mpfs/mpfs_ddr.h
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+#ifndef __ARCH_RISCV_SRC_MPFS_MPFS_DDR_H
+#define __ARCH_RISCV_SRC_MPFS_MPFS_DDR_H
+
+/****************************************************************************
+ * Public Function Prototypes
+ ****************************************************************************/
+
+#ifndef __ASSEMBLY__
+
+#undef EXTERN
+#if defined(__cplusplus)
+#define EXTERN extern "C"
+extern "C"
+{
+#else
+#define EXTERN extern
+#endif
+
+EXTERN int mpfs_ddr_init(void);
+
+#if defined(__cplusplus)
+}
+#endif
+#undef EXTERN
+
+#endif /* __ASSEMBLY__ */
+#endif /* __ARCH_RISCV_SRC_MPFS_MPFS_DDR_H */