drivers: clock_control: set voltage scaling before PLL

Move the call to prepare_regulator_voltage_scale()
before PLL setup in clock_stm32_ll_h7.c.

This ensures the voltage regulator is configured
to the appropriate voltage scale before increasing
the system clock frequency via PLLs.
Without this change, the system configuration
may be out of spec.

Signed-off-by: IBEN EL HADJ MESSAOUD Marwa <marwa.ibenelhadjmessaoud-ext@st.com>