drivers: rtc: api: Add helper for determining calibration

Add helper function which calculates the required calibration to
1 Hertz from the actual frequency of an RTC. This helper is both
functional and adds to the documentation of the RTC calibration
API.

The rtc.h header now includes zephyr/kernel.h instead of
zephyr/types.h as __ASSERT_NO_MSG is now referenced in the header.

The commit additionally extends the rtc_api_helpers test suite
with a test to validate the conversion function itself.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>