This commit enables STM32Cube LL based driver for stm32f4 series.
This generic driver provides a unified API to clock driver for all
stm32 series.
LL API allows driver to be lightweight and to keep genericity across
stm32 family to ease further devlopment and maintenance.
Change-Id: Ie31ae8f433313787f9c9eda77de41925721d54dd
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
