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/bcm7445_defconfig b/configs/bcm7445_defconfig
index 0ae0595..2c71a5e 100644
--- a/configs/bcm7445_defconfig
+++ b/configs/bcm7445_defconfig
@@ -14,6 +14,7 @@
 CONFIG_PREBOOT="fdt addr ${fdtcontroladdr};fdt move ${fdtcontroladdr} ${fdtsaveaddr};fdt addr ${fdtsaveaddr};"
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot>"
+CONFIG_CMD_MMC=y
 CONFIG_CMD_SF_TEST=y
 CONFIG_CMD_SPI=y
 CONFIG_OF_PRIOR_STAGE=y