pinctrl: update document for pinconf_generic_parse_dt_config

The returned configs from pinconf_generic_parse_dt_config() is
duplicated from original. Make it clear it must be freed when no
longer necessary.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>