dt/bindings: Document pinctrl-ingenic

This commit adds documentation for the devicetree bindings of the
pinctrl-ingenic driver, which handles pin configuration and pin
muxing of the Ingenic SoCs currently supported by the Linux kernel.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>