pinctrl: sunxi: remove struct sunxi_gpio

pinctrl: sunxi: remove struct sunxi_gpio

So far every Allwinner SoC used the same basic pincontroller/GPIO
register frame, and just differed by the number of implemented banks and
pins, plus some special functionality from time to time. However the D1
and successors use a slightly different pinctrl register layout.
Use that opportunity to drop "struct sunxi_gpio", that described that
MMIO frame in a C struct. That approach is somewhat frowned upon in the
Linux world and rarely used there, though still popular with U-Boot.

Switching from a C struct to a "base address plus offset" approach allows
to switch between the two models more dynamically, without reverting to
preprocessor macros and #ifdef's.

Model the pinctrl MMIO register frame in the usual "base address +
offset" way, and replace a hard-to-parse CPP macro with a more readable
static function.
All the users get converted over. There are no functional changes at
this point, it just prepares the stages for the D1 and friends.

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