soc: nrf53: Tune capacitor for LFXO for nRF53 and update accuracy

Switch from using internal 6pf to internal 7pf load capacitor for LFXO.
Use a default clock accuracy of 50PPM as this matches lab results.

50PPM should be sufficient for common operating temperatures of
25degC +- 15.

Signed-off-by: Johan Stridkvist <johan.stridkvist@nordicsemi.no>