dt-bindings: adreno: Add property to limit number of pages per GPU pool

This change adds documentation for GPU memory pool property
"qcom,mempool-max-pages" which can be specified for each pool.
This property provides an option to limit number of pages that
can be pooled in a given pool.

Change-Id: I56daf6bda43e0b0af73f24594869bf394a93dc7e

bindings/gpu/adreno.txt

@@ -258,6 +258,8 @@ Properties:
 - qcom,mempool-reserved:	Number of pages reserved at init time for a pool.
 - qcom,mempool-allocate:	Allocate memory from the system memory when the
 				reserved pool exhausted.
+- qcom,mempool-max-pages:	Limit on max pages this pool can hold, If not defined
+				there is no limit.
 GPU CX Ipeak:
 - qcom,gpu-cx-ipeak:		Container for GPU CX ipeak clients. Multiple clients
 				can be defined within qcom,gpu-cx-ipeak. Each client