arm: am57xx: cl-som-am57x: support for FS boot

Supported boot devices are raw QSPI and raw SD card.
Add support for a FAT16/32 file system for SD card.

The SOC's boot ROM only supports FAT file system.
Therefore remove the SPL support for the EXT file system.

Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>