dt-bindings: msm/disp: add documentation for CWB/LUTDMA QoS setting

Add documentation for few parameters required for the QoS
remap setting for CWB and LUTDMA.

Change-Id: I441600e1de2397ee1b5cc709db66ec3ec5b62c63
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>