drivers: gpio: mmio32: update gpio_mmio32 to behave like other divers

The current implementation requires SoCs/Boards to manualy instantiate
the preripherals and initilize them.

The change lets Zephyr rely on the device tree setup to instantiate &
initialize the relevant gpio peripheral.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>