drivers: clock_control: nrf: add SYNTH LFCLK clock source

Added option to have LFCLK synthesized from HFCLK. It is not low
power but ensures constant relation between HFCLK and LFCLK and
might be useful in certain scenarios (e.g. testing).

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