drivers: clock_control: Add missing dependency in Kconfig

Calibration process can be simplified if LF clock is always on.
Kconfig was depending on RTC being used as system clock because
that indicates LF clock being always on. Same can be done for
case when GRTC is used as system clock.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>