The TCONs on A31/A31s can select either backend as its input. As there
is no configurable mux in the backend or DRC to redirect their output,
or for the DRC to select an input, the connections are presumably from
the each DRC to each TCON, with the TCON having two input ports, like
the following diagram:

	Backend 0  -------  DRC 0  ------- [0]  TCON 0
				   --   -- [1]
                                     \ /
				      X
				     / \
				   --   -- [0]
	Backend 1  -------  DRC 1  ------- [1]  TCON 1

Add these connection endpoints to the device tree.
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>