ARM: dts: add Protonic VT7 board

The Protonic VT7 is a mid-class ISObus Virtual Terminal with a 7 inch
touchscreen display.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robin van der Gracht <robin@protonic.nl>
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>