ARM: dts: msm: add system PM device binding for SDM640

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

Change-Id: I154d2ad84cbc626b010a6837f08c9e42732116f7
Signed-off-by: Srinivas Rao L <lsrao@codeaurora.org>