pinctrl: nsp: add gpio-a driver support for Broadcom NSP SoC

This adds the initial support of the Broadcom NSP gpio-a driver.

Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yrdreddy@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>