gpio: virtio: Add IRQ support

This patch adds IRQ support for the virtio GPIO driver. Note that this
uses the irq_bus_lock/unlock() callbacks, since those operations over
virtio may sleep.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>