ARM: shmobile: r8a7794: Add DU node to device tree

Add the DU device with a disabled state. Boards that want to enable the
DU need to specify the output topology.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>