stm32h7xx: system clock stalls above 350Mhz

[jnaulet]

The stm32h743 is rated for 400Mhz but when i try to switch the sys_ck to a 400Mhz pll1_p_ck, the system stalls.

[jnaulet]

By playing with the voltage scaling & overdrive in PWR & SYSCFG_PWRCR, we can achieve 350Mhz, but 400Mhz is still unstable and stalls or resets.

It probably needs fine tuning, the pll computation might be slightly off & create this problem, for example.