dt-bindings: drm/msm/gpu: Document a5xx / a6xx zap shader region

Describe the zap-shader node that defines a reserved memory region
to store the zap shader.
Signed-off-by: NJordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: NRob Clark <robdclark@chromium.org>

Documentation/devicetree/bindings/display/msm/gpu.txt

@@ -27,6 +27,9 @@ Required properties:
 - qcom,gmu: For GMU attached devices a phandle to the GMU device that will
   control the power for the GPU. Applicable targets:
     - qcom,adreno-630.2
+- zap-shader: For a5xx and a6xx devices this node contains a memory-region that
+  points to reserved memory to store the zap shader that can be used to help
+  bring the GPU out of secure mode.
 
 Example 3xx/4xx/a5xx:
 
@@ -75,5 +78,9 @@ Example a6xx (with GMU):
 		interconnects = <&rsc_hlos MASTER_GFX3D &rsc_hlos SLAVE_EBI1>;
 
 		qcom,gmu = <&gmu>;
+
+		zap-shader {
+			memory-region = <&zap_shader_region>;
+		};
 	};
 };