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/cmd/Kconfig b/cmd/Kconfig
index b6e2ac3..67890c7 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -709,6 +709,15 @@
 
 endif # CMD_NAND
 
+config CMD_MMC_SPI
+	bool "mmc_spi - Set up MMC SPI device"
+	help
+	  Provides a way to set up an MMC (Multimedia Card) SPI (Serial
+	  Peripheral Interface) device. The device provides a means of
+	  accessing an MMC device via SPI using a single data line, limited
+	  to 20MHz. It is useful since it reduces the amount of protocol code
+	  required.
+
 config CMD_PART
 	bool "part"
 	select PARTITION_UUIDS