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