dt/bindings: display: Add optional property node define for Mali DP500

Add optional property node 'arm,malidp-arqos-value' for the Mali DP500.
This property describe the ARQoS levels of DP500's QoS signaling.

Signed-off-by: Wen He <wen.he_1@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190910075913.17650-1-wen.he_1@nxp.com