soc: nordic: nrf54l: Clean up internal capacitance calculations.

Code responsible for internal capacitor values containted
leftover workarounds in the calculations after PS update.
Removed redundant conversions and cleaned up both code
and comments to align both LFXO and HFXO calculation.

Signed-off-by: Michał Stasiak <michal.stasiak@nordicsemi.no>