ARM: shmobile: koelsch-reference: Add DU device to DTS

The DU device has no DT bindings yet, instantiate it as a platform
device for now.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Magnus Damm <damm@opensource.se>
[horms+renesas@verge.net.au: broken out of larger patch that
                             included board changes]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>