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/board/sunxi/Kconfig b/board/sunxi/Kconfig
index 4534bb2..94a4323 100644
--- a/board/sunxi/Kconfig
+++ b/board/sunxi/Kconfig
@@ -12,6 +12,9 @@
 config SPL_LIBGENERIC_SUPPORT
 	default y
 
+config SPL_MMC_SUPPORT
+	default y
+
 # Note only one of these may be selected at a time! But hidden choices are
 # not supported by Kconfig
 config SUNXI_GEN_SUN4I
diff --git a/board/ti/common/Kconfig b/board/ti/common/Kconfig
index 8cae612..f021923 100644
--- a/board/ti/common/Kconfig
+++ b/board/ti/common/Kconfig
@@ -27,3 +27,6 @@
 
 config SPL_LIBGENERIC_SUPPORT
 	default y
+
+config SPL_MMC_SUPPORT
+	default y