drm: of: fix documentation reference

drm: of: fix documentation reference

Documentation/devicetree/bindings/graph.txt content has move directly to
the dt-schema repo in commit 4b52be0ce6ad ("dt-bindings: Remove plain
text OF graph binding").

Point to the YAML of the official repo instead of the old file.

Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250328114148.260322-1-rgallaispou@gmail.com
Signed-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>