STM32 MCU shall set voltage regulator level with respect to set clock
frequency to reach optimal power consumption.
Voltage regulator is set prior to clock setting based on configuration
from dts/overlay file. Config_regulator_voltage is set as weak in
clock_stm32_ll_common - config_regulator_voltage can be
extended to other STM32 families without need to rewrite heavily
family clock driver, default one can be still used.
Signed-off-by: Lubos Koudelka <lubos.koudelka@st.com>