uart: update stm32 uart to support LL clock control driver

After introducing STM32Cube based clock control driver for
stm32 family, update stm32 serial driver to support it.
Once supported across the whole family, a clean up will be done.

Change-Id: I7100bc699e7918c8be71d58091da1860ab734e25
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
2 files changed