ARM: dts: add support for I2SE Duckbill 2 485

This machine is based on I2SE's Duckbill 2 board and features a
RS-485 daugther board. This device is intended to be used for
e.g. home automation purposes.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Cc: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>