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/xpress_spl_defconfig b/configs/xpress_spl_defconfig
index 2cf9c0e..128c0d5 100644
--- a/configs/xpress_spl_defconfig
+++ b/configs/xpress_spl_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_EXT_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
+CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y