pmic: stm32mp_pmic: add regulator enable/disable functions

These functions will be used through the stm32mp_regulator framework.
The plat_bind_regulator function is updated to manage PMIC.

Change-Id: I7d92eb1c4765dc3048c7290a6e3e013b2c5b4203
Signed-off-by: Yann Gautier <yann.gautier@st.com>