Rename some defines containing FAT in their name to be filesystem generic

Rename some defines containing FAT in their name to be filesystem generic:
MMCSD_MODE_FAT => MMCSD_MODE_FS
CONFIG_SPL_FAT_LOAD_ARGS_NAME => CONFIG_SPL_FS_LOAD_ARGS_NAME
CONFIG_SPL_FAT_LOAD_PAYLOAD_NAME => CONFIG_SPL_FS_LOAD_PAYLOAD_NAME
CONFIG_SYS_MMC_SD_FAT_BOOT_PARTITION => CONFIG_SYS_MMC_SD_FS_BOOT_PARTITION

Signed-off-by: Guillaume GARDET <guillaume.gardet@free.fr>
Cc: Tom Rini <trini@ti.com>