pinctrl: stm32: fix memory leak issue

configs is allocated by pinconf_generic_parse_dt_config(),
pinctrl_utils_add_map_configs() duplicates configs so it can and has to
be freed to prevent memory leaks.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>