dt-bindings: regulator: add QCOM RPMh regulator bindings

Introduce bindings for RPMh regulator devices found on some
Qualcomm Technlogies, Inc. SoCs.  These devices allow a given
processor within the SoC to make PMIC regulator requests which
are aggregated within the RPMh hardware block along with requests
from other processors in the SoC to determine the final PMIC
regulator hardware state.

Signed-off-by: David Collins <collinsd@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>