clk: qcom: Add virtio clock driver

This is virtio clock frontend driver for guest virtual machine.
It operates clocks by requesting clock backend which is in hypervisor.
The frontend driver talks with backend device via the virtio interface.

Change-Id: I0cd2def134e029b7762ddec2c799ab5057568caf
Signed-off-by: Zhiqiang Tu <ztu@codeaurora.org>