dt-bindings: soc: amlogic: canvas: document support for Meson8/8b/8m2

The canvas IP on Meson8, Meson8b and Meson8m2 is similar to the one
found on GXBB and newer. The only known difference is that the older
SoCs cannot configure the "endianness".

Add a compatible string for each of the older SoCs to make sure we won't
be using unsupported features on these SoCs.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>