drm/sun4i: Add compatible string for A31/A31s TCON (timing controller)

The A31 TCON has mux controls for how TCON outputs are routed to the
HDMI and MIPI DSI blocks.

Since the A31s does not have MIPI DSI, it only has a mux for the HDMI
controller input.

This patch only adds support for the compatible strings. Actual support
for the mux controls should be added with HDMI and MIPI DSI support.

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