Convert CONFIG_SPL_MMC_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 27bb17f..d68bbf6 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_CM_FX6=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg,MX6QDL,SPL"
 CONFIG_BOOTDELAY=3