RT #90499: STM32F4X9-SOM NOR/SDRAM errata 2.8.7 fix
Copy image from NOR to SDRAM through On-Chip RAM buffer setting SDRAM to Self-Refresh on each copy. Implement flash_read16, flash_write16 and flash_write_buffer to disable/enable SDRAM on flash access. Start and immediately put SDRAM in self-refresh on start-up for flash to work correctly.