pinctrl: nsp-gpio: fix non-static functions

Fixup warnings from functions that are not exported and
therefore should be marked static. Fixes:

drivers/pinctrl/bcm/pinctrl-nsp-gpio.c:461:5:
  warning: symbol 'nsp_pin_config_group_get' was not declared.
  Should it be static?
drivers/pinctrl/bcm/pinctrl-nsp-gpio.c:467:5:
  warning: symbol 'nsp_pin_config_group_set' was not declared.
  Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Ray Jui <ray.jui@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>