MMC SD fs boot partition config coding style and proper description

CONFIG_SYS_MMC_SD_FS_BOOT_PARTITION ought to be called
CONFIG_SYS_MMCSD_FS_BOOT_PARTITION to keep it consistent with other config
options such as: CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR.

In addition, it is not related to raw mode booting but to fs mode instead.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@ti.com>