ARM: dts: add support for I2SE Duckbill 2 SPI

This machine is based on I2SE's Duckbill 2 board and is sold as part
of I2SE's PLC Bundle for IoT. This is a development kit for Homeplug
Green PHY based powerline products based on Qualcomms QCA7000 chip.

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