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/mach-uniphier/Kconfig b/arch/arm/mach-uniphier/Kconfig
index 573d942..4c7300c 100644
--- a/arch/arm/mach-uniphier/Kconfig
+++ b/arch/arm/mach-uniphier/Kconfig
@@ -6,6 +6,9 @@
 config SPL_LIBGENERIC_SUPPORT
 	default y
 
+config SPL_MMC_SUPPORT
+	default y if !ARCH_UNIPHIER_64BIT
+
 config SYS_CONFIG_NAME
 	default "uniphier"