ks2_evm: config: enable fatload command

The keystone2 evm can boot from USB partition with FAT32 FS, so
enable generic load command and fatload command usage.

Acked-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>