pinctrl: stmfx: update pin name

Update pin name to avoid duplicated name with SOC GPIO
gpio0...gpio15 / agpio0....agpio7: add a stmfx prefix.

This pin name can be used in pinmux command.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>