pinctrl: nsp: add pinmux driver support for Broadcom NSP SoC

This adds the initial support of the Broadcom NSP pinmux driver.

Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>