ARM: dts: add Protonic WD2 board

Add support for the Protonic WD2 board, which is an internal development
platform for low-cost agricultural Virtual Terminals based on COTS tablets
and web applications.
It inherits from the PRTI6Q base class.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>