gpio: dt-bindings: document the official use of "ngpios"

There are a bunch of drivers that utilize the "ngpios" DT property
without any vendor prefix. Try to start cleaning up the mess by
defining what we mean by this property.

Cc: devicetree@vger.kernel.org
Cc: Pramod Kumar <pramodku@broadcom.com>
Cc: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>