dt-bindings: display: meson-vpu: Add missing amlogic,canvas property

This property was already mentioned in the old textual bindings
amlogic,meson-vpu.txt, but got dropped during conversion.
Adding it back similar to amlogic,gx-vdec.yaml.

Fixes: 6b9ebf1e

 ("dt-bindings: display: amlogic, meson-vpu: convert to yaml")
Signed-off-by: Alexander Stein <alexander.stein@mailbox.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211219094155.177206-1-alexander.stein@mailbox.org