soc: arm: stm32h7: rework STM32H7 dual core boot

It happens that CM7 wakeups CM4, before CM4 goes to sleep.
Thus when CM4 goes to sleep,
there no more wakeup from CM7. And CM4 hangs.
For a simple synchronisation implementation,
CM4 doesn't go to sleep any more,
instead it waits (active wait) for CM7 to take HSEM
(meaning that clock configuration is finished).

Fixes #38069

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>