gpio: remove the open_drain API and ops

This patch removes the ops get_open_drain/set_open_drain
and the API dm_gpio_get_open_drain/dm_gpio_set_open_drain.

The ops only provided in one driver (mpc8xxx gpio) and the
associated API is never called in boards.

This patch prepare a more generic set/get_dir_flags ops,
including the open drain property.

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