pinctrl: at91-pio4: fix non-exported functions

The atmel_pctl_find_group_by_pin() and the atmel_pinctrl_remove()
functions are not exported, so fix the warnings about these
being exported without definitions by making them static.

drivers/pinctrl/pinctrl-at91-pio4.c:424:20: warning: symbol 'atmel_pctl_find_group_by_pin' was not declared. Should it be static?
drivers/pinctrl/pinctrl-at91-pio4.c:1077:5: warning: symbol 'atmel_pinctrl_remove' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>