dt-bindings: vendor-prefixes: add "virtual" prefix

"virtual" is used for vendor-less "devices". For example for the GPIO
based MDIO bus "virtual,mdio-gpio".

This patch is needed to fix the checkpatch warning for the Protonic WD3 board.

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