ARM: dts: msm: add system PM device binding for sdmmagpie

System PM device handles all activities supporting the SoC's
capability to enter system sleep modes. Add device bindings
for this.

Change-Id: I8cbcdfdfd074d78fb64203afd0d47f9ffeb76469
Signed-off-by: Srinivas Rao L <lsrao@codeaurora.org>