drm/sun4i: sun6i-drc: Support DRC on A31 and A31s

The A31 and A31s also have the DRC as part of the display pipeline.
As we know virtually nothing about them, just add compatible strings
for both SoCs to the stub driver.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>