xtensa/esp32: Add missing param documentation for SPI Flash function

Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>

arch/xtensa/src/esp32/esp32_spiflash.h

@@ -57,6 +57,8 @@ extern "C"
  * Input Parameters:
  *   mtd_offset - MTD Partition offset from the base address in SPI Flash.
  *   mtd_size   - Size for the MTD partition.
+ *   encrypted  - Flag indicating whether the newly allocated partition will
+ *                have its content encrypted.
  *
  * Returned Value:
  *   ESP32 SPI Flash MTD data pointer if success or NULL if fail.