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

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

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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>;

};