documentation/gpu: Add DT bindings documentation for kona

Add snapshot of missing KGSL DT bindings documentation
from msm-4.14 to kona as of commit 9ac69cf (clk: qcom: gcc:
Support code for clock controller for sdxprairie).

Change-Id: Icf0caf548e2db6ec625775c2b074064d8ca0b4fe
Signed-off-by: Urvashi Agrawal <urvaagra@codeaurora.org>