dt-bindings: gpio: convert rk3328-grf-gpio.txt to YAML

Current dts files with RK3328 GRF 'gpio' nodes are manually verified.
In order to automate this process rk3328-grf-gpio.txt has to be
converted to YAML.

Rename 'grf-gpio' nodename to 'gpio'.

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>