dt-bindings: gpio: zynq: Add power-domains

Describe optional power-domain property to fix dts_check warnings.
The similar change was done by commit 8c0aa567 ("dt-bindings: gpio:
fsl-imx-gpio: Add power-domains").

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/655523d7bf9658eb0b8e49a06c8b79a04052e5d5.1634286595.git.michal.simek@xilinx.com


Signed-off-by: Michal Simek <michal.simek@amd.com>