pinctrl: stmfx: update pincontrol and gpio device name

The device name is used in pinmux command and in log trace
so it is better to use the parent parent name ("stmfx@42" for
example) than a generic name ("pinctrl" or "stmfx-gpio")
to identify the device instance.

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