drivers: clock control: stm32h5 set the clock freq for voltage scaling

According to the stm32h5x product specifications, the values of the
voltage ragulator depends on the system clock as follows:
• VOS0 (Vcore = 1.35V) with CPU and peripherals running at up to 250 MHz
• VOS1 (Vcore = 1.2V) with CPU and peripherals running at up to 200 MHz
• VOS2 (Vcore = 1.1V) with CPU and peripherals running at up to 150 MHz
• VOS3 (Vcore = 1.0V) with CPU and peripherals running at up to 100 MHz

Signed-off-by: Francois Ramu <francois.ramu@st.com>