dt-bindings: arm: add entries for I2SE boards

This patch adds the missing devicetree binding documentation for I2SE's
Duckbill and Duckbill 2 series boards.

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