atmel: add CMD_FS_GENERIC to SAMA5D3 for EFI boot

I'm working on getting OpenBSD to run on the Atmel SAMA5D3 Xplained.
OpenBSD uses an EFI bootloader, which requires the "load" command to
work, since "fatload" doesn't initialize the EFI memory addresses
properly. To get "load" on the SAMA5D3 version of U-Boot, I propose
adding the CONFIG_CMD_FS_GENERIC option.

Signed-off-by: Greg Czerniak <greg@czerniak.info>