gpio: sunxi: Implement .set_flags

This, along with gpio_flags_xlate(), allows the GPIO driver to handle
pull-up/down flags provided by consumer drivers or in the device tree.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>