boards: stm32f4: Provide config for LL Clock control

This commit provides CONFIG_ definitions to support
LL based clock driver on stm32f4 boards:
96b_carbon
nucleo_f401re
nucleo_f411re
This commit sticks to clock configurations previously defined.
Two changes to notice:
-HSE clock value should now be defined
-Prescaler values required as "1" should now be set to "1" instead
of "0".

Change-Id: I003bd226f198217d5e266e11fe37094773c1c62c
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>