virtio_mmio: add support to set a virtio device as wakeup source

According to virtio_mmio wakeup flag in device trees, set its IRQ
as wakeup source in virtqueue initialization.

Change-Id: Id861bf10d475c66bcb343dbbb01ba143338dd18c
Signed-off-by: Minghao Xue <mingxue@codeaurora.org>