Bluetooth: Controller: nRF54Lx: Use NRF_GRTC for radio scheduling

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.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>