arm/dt: msm8974-v2: Add PM SNOC bus client device

Add an SNOC bus client device for PM. This will mitigate long
exit latencies out of power collapse by removing the bus limitation
for TZ code.

The bus request is only in place when Apps is active, an RPM notified
sleep will transition Apps to its sleep set and remove the bus request.

Change-Id: I247c738f708d005d516c2bec62c26964f589ae52
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>