mx6ul_14x14_evk: Remove CONFIG_SPL_FAT_SUPPORT

If the SD card does not contain the u-boot.img then we get the
following error:

U-Boot SPL 2015.10-rc2-23947-g7ad5930 (Sep 08 2015 - 14:10:29)
** Partition 1 not valid on device 0 **
spl_register_fat_device: fat register err - -1
spl_load_image_fat: error reading image u-boot.img, err - -1

Remove CONFIG_SPL_FAT_SUPPORT and let CONFIG_SPL_MMC_SUPPORT do the
job.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>