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/etamin_defconfig b/configs/etamin_defconfig
index 2805ff2..d7b924e 100644
--- a/configs/etamin_defconfig
+++ b/configs/etamin_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
+CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-draco"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y