dt-bindings: display: amlogic, meson-vpu: Add optional canvas provider node

Allows using the new canvas provider module if present.
Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181105104508.23090-2-mjourdan@baylibre.com

Documentation/devicetree/bindings/display/amlogic,meson-vpu.txt

@@ -67,6 +67,8 @@ Required properties:
 Optional properties:
 - power-domains: Optional phandle to associated power domain as described in
 	the file ../power/power_domain.txt
+- amlogic,canvas: phandle to canvas provider node as described in the file
+	../soc/amlogic/amlogic,canvas.txt
 
 Required nodes: