pinctrl: artpec6: Make two functions static

Fix sparse warnings:

drivers/pinctrl/pinctrl-artpec6.c:691:5: warning:
 symbol 'artpec6_pmx_enable' was not declared. Should it be static?
drivers/pinctrl/pinctrl-artpec6.c:705:6: warning:
 symbol 'artpec6_pmx_disable' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>