gpio: sandbox: Rename GPIO dir_flags to flags

Adjust the terminology in this driver to reflect that fact that all flags
are handled, not just direction flags.

Create a new access function to get the full GPIO state, not just the
direction flags. Drop the static invalid_dir_flags since we can rely on a
segfault if something is wrong.

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