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

According to virtio_mmio wakeup flag in device tree, record devices to a
list in platform device probing and then set its IRQ as wakeup source in
virtqueue initialization.

Change-Id: Ic7b1d4ed283d1d8f28c035ef8663c4e392e6dacb
Signed-off-by: Yimin Peng <yiminp@codeaurora.org>