power: pmic: stpmu1: Add regulator bindings

Add regulator bindings to get access to regulator managed
by drivers/power/regulator/stpmu1.c regulator driver.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>