clock_control: Cleanup stm32f10x 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: Idaaa0d705a31bc69cdf7e576e303f581d3d0bd5f
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>