virtio_mmio: Change initcall level to arch_initcall

Virtio clock depends on virtio mmio. Initcall level is changed to
make sure that virtio_mmio is ready before virtio clock probe.

Change-Id: I174d1c85e88d0e124dd07bce57c38d2b5318de52
Signed-off-by: Zhiqiang Tu <ztu@codeaurora.org>