stm32: update the timer pre-scaler on STM32F4
stm32: update the timer pre-scaler on STM32F4

Add support for changing for the timer source clock frequency on
STM32F4 by re-computing the timer pre-scaler value in the
HOOK_FREQ_CHANGE callback.
Preparatory work to implement power management switching between PLL and
HSI as sysclk.

Signed-off-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

BUG=b:130561737
TEST=manual, verify the output of the 'gettime' console command against
the wall clock.
BRANCH=fpmcu-bloonchipper

Change-Id: Ia7dc24d3b1dbff816b1cebbc87492e678b987dfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2527048
Tested-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Commit-Queue: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec