dt-bindings: gpio: zynq: convert bindings to YAML

Convert gpio for Xilinx Zynq SoC bindings documentation to YAML.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>