ARM: dts: imx6: add ZII RDU2 boards

This adds support for the Zodiac Inflight Innovations RDU2 board,
which has both a Quad and a QuadPlus variant.

The board supports different panels, with the bootloader patching
in the correct compatible, depending on the hardware configuration.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>