mmc: sunxi: Document host init sequence

sunxi_mmc_init_host() originated from Allwinner kernel sources. The
magic numbers written to various registers was never documented.

Add comments for values found in Allwinner user manuals.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>