Qualcomm REFGEN regulator

Merge series from Konrad Dybcio <konrad.dybcio@linaro.org>:

Recent Qualcomm SoCs have a REFGEN (reference voltage generator) regulator
responsible for providing a reference voltage to some on-SoC IPs (like DSI
or PHYs). It can be turned off when unused to save power.

This series introduces the driver for it.