soc: qcom: subsystem_notif_virt: Add virtual subsystem notification driver

The guest VM uses this driver to communicate subsystem state related
notifications to a backend driver via the virtual device's registers.

Change-Id: I612fcb641c4d531c1d2c0fd18f44f7ebff040f2c
Signed-off-by: Anant Goel <anantg@codeaurora.org>