drivers: clock_control: stm32f7 flash latency with overdrive

The Flash latency depends on the sysclock
In case of the stm32F7 the regulator overdrive mode is set
depending on the sys clock freq.
The overdrive must be set before the first LL_SetFlashLatency.

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