dt-bindings: pinctrl: Add DT bindings for apple, pinctrl

The Apple GPIO controller is a simple combined pin and GPIO conroller
present on Apple ARM SoC platforms, including various iPhone and iPad
devices and the "Apple Silicon" Macs.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Link: https://lore.kernel.org/r/20210520171310.772-2-mark.kettenis@xs4all.nl
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>