mfd: devicetree: bindings: Add Qualcomm RPM DT binding

Add binding for the Qualcomm Resource Power Manager (RPM) found in 8660,
8960 and 8064 based devices.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>