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/picosam9g45_defconfig b/configs/picosam9g45_defconfig
index 61acc7c..eb0fdee 100644
--- a/configs/picosam9g45_defconfig
+++ b/configs/picosam9g45_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
+CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9M10G45,SYS_USE_MMC"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y