pmic: add stpmu1 support

This driver implements register read/write operations for STPMU1.

The STPMU1 PMIC provides 4 BUCKs, 6 LDOs, 1 VREF
and 2 power switches. It is accessed via an I2C interface.
This device is used with STM32MP1 SoCs.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>