mmc: sunxi: Honour non-removable property in DT

If a board DT describes a cd-gpios property, but also marks the storage
as non-removable, we must ignore the GPIO (as Linux does).

Teach the DM_MMC part of the Allwinner MMC driver about the
non-removable DT property, to fix DM_MMC access on the SoPine and
Pine64-LTS board.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Jagan Teki <jagan@openedev.com>