drivers: gpio: add VIRTIO GPIO driver

Add a driver for the VIRTIO GPIO device (virtio spec 1.3, section 5.18),
letting a Zephyr guest drive GPIO lines exposed by a hypervisor or a
vhost-user backend.

The number of lines is read from the device configuration space at init,
and interrupts use the event virtqueue when the device offers
VIRTIO_GPIO_F_IRQ. Every operation is a round trip to the device, so the
GPIO API can only be used from a thread.

Assisted-by: Claude Code:opus-4.8
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>