pinctrl: Add new compatible string to GPIO controller driver

This compatible string should be used for all new iproc based future
SoCs having the same GPIO controller hardware.

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>