dt-bindings: gpio: Convert apm,xgene-gpio-sb to DT schema

Convert APM X-Gene Standby GPIO binding to DT schema format. It's a
straight forward conversion.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250714202821.3011099-1-robh@kernel.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>