dt-bindings: display: amlogic, meson-vpu: Add optional canvas provider node

Allows using the new canvas provider module if present.

Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181105104508.23090-2-mjourdan@baylibre.com