spl: mmc: Allow to disable SYS_MMCSD_FS_BOOT_PARTITION

On some platforms is SYS_MMCSD_FS_BOOT_PARTITION unsupported. So allow to
completely disable MMC FS Boot support via new option SYS_MMCSD_FS_BOOT.

By default MMC FS Boot support is enabled (like it was before) except for
ARCH_MVEBU where MMC FS Boot supported is unsupported due to Marvell
BootROM limitations.

Signed-off-by: Pali Rohár <pali@kernel.org>