Qualcomm GPU powerlevels

Powerlevels are defined in sets by qcom,gpu-pwrlevels. Multiple sets (bins)
can be defined within qcom,gpu-pwrelvel-bins. Each powerlevel defines a
voltage, bus, and bandwitdh level.

- qcom,gpu-pwrlevel-bins:	Contains one or more qcom,gpu-pwrlevels sets

Properties:
- compatible:			Must be qcom,gpu-pwrlevel-bins
- qcom,gpu-pwrlevels:		Defines a set of powerlevels

Properties:
- qcom,speed-bin:		Speed bin identifier for the set - must match
				the value read from the hardware

- qcom,gpu-pwrlevel:		A single powerlevel

Properties:
- reg:				Index of the powerlevel (0 = highest perf)
- qcom,gpu-freq			GPU frequency for the powerlevel (in Hz)
- qcom,bus-freq			Index to a bus level (defined by the bus
				settings)
- qcom,bus-min			Minimum bus level to set for the power level
- qcom,bus-max			maximum bus level to set for the power level