dt-bindings: sun6i-dsi: Add A64 DPHY compatible (w/ A31 fallback)

The MIPI DSI PHY controller on Allwinner A64 is similar
on the one on A31.

Add A64 compatible and append A31 compatible as fallback.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191222132229.30276-3-jagan@amarulasolutions.com