[nrf52]calibrated_rc_sleep_clock HFXO on-time should be optimized

[TorstenRobitzki]

Currently, the time, the HFXO is switched on for the periodical RC sleep clock calibration, is synchronized to full connection event length (or advertising).

When the calibrated sleep clock is used, it should be possible, to check whether it is save, to disable the HFXO until it is switched on again. Alternatively, there must be some kind of atomic HFXO usage count, implemented in software.