devicetree: soc: Add Qualcomm SMD based RPM DT binding

Add binding documentation for the Qualcomm Resource Power Manager (RPM)
using shared memory (Qualcomm SMD) as transport mechanism. This is found
in 8974 and newer based devices.

The binding currently describes the rpm itself and the regulator
subnodes.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Andy Gross <agross@codeaurora.org>