Convert CONFIG_CMD_MMC_SPI to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_MMC_SPI

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/microblaze-generic_defconfig b/configs/microblaze-generic_defconfig
index 12dcd00..6233803 100644
--- a/configs/microblaze-generic_defconfig
+++ b/configs/microblaze-generic_defconfig
@@ -23,6 +23,7 @@
 CONFIG_SYS_PROMPT="U-Boot-mONStR> "
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GPIO=y
+CONFIG_CMD_MFSL=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_TFTPPUT=y
 CONFIG_CMD_DHCP=y