dt-bindings: add BCM6318 pincontroller binding documentation

Add binding documentation for the pincontrol core found in BCM6318 SoCs.

Co-developed-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210324081923.20379-21-noltari@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>