dt-bindings: arm: fsl: add different Protonic boards

Add Protonic PRTI6Q, WD2, RVT, VT7 boards.

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