dt-bindings: display: add sde rsc sub device node compatible string

Add compatible string documentation for sde rsc sub device
present in display rsc.

Change-Id: I3cb94c1b1a48cf20e81bfa0a7389a0f87d2f6b5b
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>