drivers: clock_control: nrf: Improve waiting for LF clock

Enabled going to idle when waiting for low frequency clock.
Added 2 stages of starting LF clock when XTAL is used. First
stage is starting RC and then when it is ready XTAL is started.
It is done to get event/interrupt when RC is ready which means
that LF clock is available (but not stable).

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