drivers: clock_control: stm32u5 enables the EPOD

With the stm32U5, when the sysclock is > 55 MHz, the EPOD booster
must be configured before the PLL1 is enabled (see refMan).
This is the case when sysclock is on PLL1 sourced by MSIS or
HSE higher than 16MHz.

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