ARM: dts: imx28-duckbill-2: Include base board

All additional I2SE Duckbill 2 variants always have the same
base board in common. So consider this by including the base
board and avoid a lot of redundancy.

Special care needs to be taken of the SPI variant. ssp2 is used
as SD card interface on the base board, but on the SPI variant
it's downgrade to a SPI interface to connect the QCA7000. So
the SD card properties must be deleted.

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