dt-bindings: arm: msm: Add sleepstate-smp2p bindings

The smp2p sleepstate driver is used to notify remote processors of the
power state changes on the local processor. This driver also receives
notifications from remote processors to prevent suspend through smp2p.

Change-Id: I5fdea66a2bdc7cfba75fc2be8eee8820ceb965bf
Signed-off-by: Chris Lew <clew@codeaurora.org>