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/am335x_shc_defconfig b/configs/am335x_shc_defconfig
index 00f682e..5bebd9b 100644
--- a/configs/am335x_shc_defconfig
+++ b/configs/am335x_shc_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_SERIES=y
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_FIT=y