dt-bindings: display: adi,adv75xx: Add missing graph schema references

DT bindings using the graph binding must have references to the graph
binding schema. These are missing from the adi,adv7511 and adi,adv7533
bindings, so add them.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20220823145649.3118479-12-robh@kernel.org