configs: omapl138_lcdk: dont disable fat write

CONFIG_FAT_WRITE is imply'ed when CONFIG_CMD_FAT
is selected (see CONFIG_TI_COMMON_CMD_OPTIONS).

Dont disable it in defconfig so the imply takes
effect and 'fatwrite' is available for users.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>