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>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 3c2af45..2c042ad 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -816,8 +816,17 @@
 	  this option to build the drivers in drivers/mmc as part of an SPL
 	  build.
 
+config SYS_MMCSD_FS_BOOT
+	bool "MMC FS Boot mode"
+	depends on SPL_MMC
+	default y if !ARCH_MVEBU
+	help
+	  Enable MMC FS Boot mode. Partition is selected by option
+	  SYS_MMCSD_FS_BOOT_PARTITION.
+
 config SYS_MMCSD_FS_BOOT_PARTITION
 	int "MMC Boot Partition"
+	depends on SYS_MMCSD_FS_BOOT
 	default 1
 	help
 	  Partition on the MMC to load U-Boot from when the MMC is being