dt-bindings: gpio: stp: convert to json-schema

Convert the Lantiq STP Device Tree binding documentation to json-schema.
Add the missing pinctrl property to the example. Add missing lantiq,phy3
and lantiq,phy4 bindings for xRX300 and xRX330 SoCs.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>