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/gwventana_defconfig b/configs/gwventana_defconfig
index d3366e5..d5d4f68 100644
--- a/configs/gwventana_defconfig
+++ b/configs/gwventana_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_GW_VENTANA=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_STACK_R_ADDR=0x18000000
 CONFIG_DM_SERIAL=y
 CONFIG_FIT=y