dt-bindings: add BCM6318 GPIO sysctl binding documentation

Add binding documentation for the GPIO sysctl found in BCM6318 SoCs.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210324081923.20379-22-noltari@gmail.com


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