msm: kgsl: Add missing DT bindings documentation

Add snapshot of missing KGSL DT bindings documentation
from msm-4.9 kernel as of commit e023d014d203 
(msm: pcie:fix PCIe L0s and L1 ASPM support).


Change-Id: Ie32094caa6613bc27f0cab5a1fe49f20fc335e29
Signed-off-by: Urvashi Agrawal <urvaagra@codeaurora.org>