drivers: pinctrl: add driver for Allwinner A64 SoC

Based on the Allwinner A64 user manual and on the previous sunxi
pinctrl drivers this introduces the pin multiplex assignments for
the ARMv8 Allwinner A64 SoC.
Port A is apparently used for the fixed function DRAM controller, so
the ports start at B here (the manual mentions "n from 1 to 7", so
not starting at 0).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>