Bluetooth: Controller: nRF54Lx: Use NRF_GRTC for radio scheduling

[cvinayak]

Use NRF_GRTC, Global real-time counter, instead of NRF_RTC.

NRF_GRTC is present in a different power domain in the SoC. Also, NRF_GRTC provide microsecond resolution timing units, in the future use of NRF_TIMER for packet timer can also be replaced.

Use of NRF_RTC would keep Radio power domain ON in addition to already ON NRF_GRTC's power domain, hence switch to using NRF_GRTC on nRF54Lx SoC to have lower power consumptions.

Add ticker implementation support for free running counter use.