clock_control: Cleanup stm32f4x driver

This patch addresses the following issues:
* Aligns the Kconfig code style with Zephyr projects requirements.
* Removes redundant "depends on" from Kconfig.
* Adds static to the containing file scope declarations.

Change-Id: I14651826724c014cc71d62ab6cab03c668c578c0
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>