dt-bindings: gpio-mmio: Add MMIO for IXP4xx expansion bus

dt-bindings: gpio-mmio: Add MMIO for IXP4xx expansion bus

This expansion is a simple MMIO-mapped GPIO device but the bus has a
number of additional properties that we need to bring in using
a reference to the bus child node schema.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250822-ixp4xx-eb-mmio-gpio-v2-2-bd2edd4a9c74@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>