dt-bindings: Add a prefix for Gateworks Corporation

As I am adding a PLD GPIO driver for Gateworks, we need
their prefix among the vendors.

Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>