documentation/gpu: Add throttle-pwrlevel optional property for adreno GPU

Optional Properties:

- qcom,initial-powerlevel: This value indicates which qcom,gpu-pwrlevel should be used at start time

			   and when coming back out of resume

- qcom,throttle-pwrlevel:  This value indicates which qcom,gpu-pwrlevel LM throttling

			   may start to occur

- qcom,bus-control:	   Boolean. Enables an independent bus vote from the gpu frequency

- qcom,bus-width:	   Bus width in number of bytes. This enables dynamic AB bus voting based on

			   bus width and actual bus transactions.