pinctrol: dm: remove the function pinctrl_decode_pin_config

Remove the pinctrl_decode_pin_config() API, because this
function is unused and not compatible with livetree
(it uses fdtdec_get_bool instead of ofnode API).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>