arm: atmel: sama5d3: spl boot from fat fs SD card

Enable Atmel sama5d3xek boart spl boot support, which can load u-boot
from SD card with FAT file system.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>