dt-bindings: gpio: add YAML description for rockchip,gpio-bank

Current dts files with "rockchip,gpio-bank" subnodes
are manually verified. In order to automate this process
the text that describes the compatible in rockchip,pinctrl.txt
is removed and converted to YAML in rockchip,gpio-bank.yaml.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>