power: regulator: stpmu1: Introduce stpmu1 driver

Enable support for the regulator functions of the STPMU1X PMIC. The
driver implements get/set api for the various BUCKS and LDOs supported
by the PMIC device. This driver is controlled by a device tree node
which includes voltage limits.

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>