sunxi: Use Kconfig CONFIG_MMC

Not all sunxi boards have an MMC embedded. Switching to the Kconfig option
will allow to enable or disable the support in each boards' defconfig.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
diff --git a/board/sunxi/Kconfig b/board/sunxi/Kconfig
index b336777..f6f2a60 100644
--- a/board/sunxi/Kconfig
+++ b/board/sunxi/Kconfig
@@ -227,6 +227,10 @@
 	Set this to enable various workarounds for old kernels, this results in
 	sub-optimal settings for newer kernels, only enable if needed.
 
+config MMC
+	depends on !UART0_PORT_F
+	default y if ARCH_SUNXI
+
 config MMC0_CD_PIN
 	string "Card detect pin for mmc0"
 	default ""