clock control:stm32: provide STM32Cube LL based driver

This new clock control driver aims at providing a lightweight
generic driver to the whole stm32 family.
Driver is based on LL Cube API and hence is for now available
only for STM32 supporting this API: stm32f3xx and stm32l4xx.
Other families should be supported soon.
Once globally supported, this unique driver will also help
reducing the impact of stm32 clock control heterogeneity in
other drivers.

Change-Id: Id15a8d0d44f03809b2907ef10d6877700459e674
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>