gpio: Rename dm_gpio_get_dir_flags() to dm_gpio_get_flags()

This function can be used to get any flags, not just direction flags.
Rename it to avoid confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>