ARM: dts: porter: add DU DT support

Define  the Porter board dependent part of the DU device node.
Add the device node  for Analog Devices ADV7511W  HDMI transmitter to I2C2
bus and the HDMI connector. Add the necessary subnodes to interconnect  DU
and HDMI devices.

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