ARM: shmobile: r8a7790: 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>
[horms+renesas@verge.net.au: resolved minor conflict]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>