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>