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>