ARM: dts: add Protonic RVT board

Protonic RVT is an internal development platform for a wireless ISObus
Virtual Terminal based on COTS tablets, and the predecessor of the WD2
platform.

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>