sunxi: pinctrl: Create the driver skeleton

Create a do-nothing driver for each sunxi pin controller variant.

Since only one driver can automatically bind to a DT node, since the
GPIO driver already requires a manual binding process, and since the
pinctrl driver needs access to some of the same information, refactor
the GPIO driver to be bound by the pinctrl driver. This commit should
cause no functional change.

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