dt-bindings: improve BCM6345 GPIO binding documentation

dt-bindings: improve BCM6345 GPIO binding documentation

Convert existing BCM6345 GPIO binding documentation to YAML and add binding
documentation for the GPIO controller found in BCM6318, BCM6328, BCM6358,
BCM6362, BCM6368 and BCM63268 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/20210324081923.20379-4-noltari@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>