dt-bindings: soc: amlogic: add meson-canvas documentation

DT bindings doc for amlogic,meson-canvas
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>

Documentation/devicetree/bindings/soc/amlogic/amlogic,canvas.txt

+Amlogic Canvas
+================================
+
+A canvas is a collection of metadata that describes a pixel buffer.
+Those metadata include: width, height, phyaddr, wrapping, block mode
+and endianness.
+
+Many IPs within Amlogic SoCs rely on canvas indexes to read/write pixel data
+rather than use the phy addresses directly. For instance, this is the case for
+the video decoders and the display.
+
+Amlogic SoCs have 256 canvas.
+
+Device Tree Bindings:
+---------------------
+
+Video Lookup Table
+--------------------------
+
+Required properties:
+- compatible: "amlogic,canvas"
+- reg: Base physical address and size of the canvas registers.
+
+Example:
+
+canvas: video-lut@48 {
+	compatible = "amlogic,canvas";
+	reg = <0x0 0x48 0x0 0x14>;
+};