Documentation: dts: add bindings for Vybrid GPIO/PORT module

The Vybrid SoC device tree (vf610.dtsi) used this bindings since
its initial commit in May 2013. However, a proper gpiolib driver
was missing so far. With the addition of the gpiolib driver,
the bindings proved to be useful and complete, hence a good time
to add the documentation.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>