dts: stm32: Add rcc prop undershoot-prevention

On some parts,  it could be required to use steps before applying
highest frequencies.
This was previously done as part of LL_PLL_ConfigSystemClock_FOO
utility functions which are no more used.
Use device tree to mention when this is required and implement it
in stm32_clock_control_init().

Additionally, fix the calls tp LL_RCC_SetAHBPrescaler, which require
use of ahb_prescaler helper.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>