ARM: dts: wheat: add DU support

Define  the  Wheat board dependent  part of the DU device node.
Add the device nodes for the Analog Devices ADV7513 HDMI transmitters
connected to DU0/1.  Add the necessary subnodes to interconnect DU with
HDMI transmitters/connectors.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>