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/arch/arm/cpu/armv7/omap5/Kconfig b/arch/arm/cpu/armv7/omap5/Kconfig
index ff9ad6e..e74a867 100644
--- a/arch/arm/cpu/armv7/omap5/Kconfig
+++ b/arch/arm/cpu/armv7/omap5/Kconfig
@@ -21,6 +21,9 @@
 config SPL_LIBGENERIC_SUPPORT
 	default y
 
+config SPL_MMC_SUPPORT
+	default y
+
 choice
 	prompt "OMAP5 board select"
 	optional