clock_control: Cleanup quark_se 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: I6d48d2eaf6ffd5fa28b37e1d3ca2d467705110f3
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>