Convert CONFIG_CMD_MMC to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_MMC

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx8mq_phanbell_defconfig b/configs/imx8mq_phanbell_defconfig
index 651940c..045d204 100644
--- a/configs/imx8mq_phanbell_defconfig
+++ b/configs/imx8mq_phanbell_defconfig
@@ -21,6 +21,7 @@
 # CONFIG_BOOTM_NETBSD is not set
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
+CONFIG_CMD_MMC=y
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_REGULATOR=y
 CONFIG_CMD_EXT2=y