mmc: sunxi: Use DM_GPIO flags to set pull-up

Now that the sunxi_gpio driver handles pull-up/down via the driver
model, pin configuration does not need a platform-specific function.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>