mmc: sunxi: Introduce a sunxi_mmc_cfg struct

Create a struct to hold the various model / compatible string dependend
settings.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>