board: sama5d3_xplained: add Falcon boot support

board: sama5d3_xplained: add Falcon boot support

This supports Falcon boot support for Microchip SAMA5D3 Xplained,
tested on raw MMC, and on raw NAND.

spl_start_uboot() is has the simplest possible implementation.
It doesn't test the environment because enabling environment support
currently causes the SPL to exceed its maximum size (64 KiB).
It doesn't check the serial for incoming characters either because
this functionality currently doesn't seem to work from the SPL
on this board.

Settings for Falcon boot from at FAT partition are also added to
avoid compile failures when CONFIG_SPL_OS_BOOT is enabled, but this
particular case is currently not functional as adding FAT and
partition support cause the SPL to be too big again.

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>