pinctrl: Cygnus: define Broadcom Cygnus GPIO/PINCONF binding

Document the GPIO/PINCONF device tree binding for Broadcom Cygnus SoC

Signed-off-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>