pinctrl: lantiq: Staticize non-exported symbols

Both ltq_pinctrl_dt_node_to_map() and ltq_pinctrl_dt_free_map() are not
referenced outside of this file. Make them static.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>