doc: Add gpio status output fields description

Add gpio status output fields description and one output example.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Tweak the formatting.
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
1 file changed