pinctrl: Rename gpio driver from cygnus to iproc

Rename gpio driver file name from pinctrl-cygnus-gpio.c to
pinctrl-iproc-gpio.c to make it more generic so that all
iproc based future SoCs using the same gpio block could
use this driver.

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