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/drivers/mmc/Kconfig b/drivers/mmc/Kconfig
index d3d7d91..ceae7bc 100644
--- a/drivers/mmc/Kconfig
+++ b/drivers/mmc/Kconfig
@@ -2,7 +2,7 @@
 
 config MMC
 	bool "Enable MMC support"
-	depends on UNUSED
+	depends on ARCH_SUNXI
 	help
 	  TODO: Move all architectures to use this option