dt-bindings: display: amlogic, meson-vpu: Add optional power domain property

The Video Processing Unit power domain was setup by the Vendor U-Boot,
add support for an optional Power Domain phandle to setup it from the kernel.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1512561268-29806-2-git-send-email-narmstrong@baylibre.com