drivers: timer: nrf_rtc_timer: do not start LFCLK unless CLOCK_CONTROL

Do not start the LF clock unless CONFIG_CLOCK_CONTROL_NRF is enabled.
This is similar to how it's done in Nordic's GRTC driver.

Authored-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
Signed-off-by: Emanuele Di Santo <emdi@nordicsemi.no>