drivers: clock_control: nrf: Switch to single clock device

Low frequency and high frequency clocks had separate devices
while they are actually handled by single peripheral with single
interrupt. The split was done probably because opaque subsys
argument in the API was used for other purposes and there was
no way to pass the information which clock should be controlled.
Implementation changes some time ago and subsys parameter was
no longer used. It now can be used to indicate which clock should
be controlled.

Change become necessary when nrf5340 is taken into account where
there are more clocks and current approach would lead to create
multiple devices - mess.

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