pinctrl: cygnus: Add new compatible string for gpio controller driver

This new compatible string, "brcm,iproc-gpio", should be used for
all new iproc-based future SoCs.

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