drivers: timer: nrf_grtc: Add GRTC fix for app and rad cores

Currently function `z_nrf_grtc_wakeup_prepare()` should be available
only for the GRTC manager (`CONFIG_NRF_GRTC_START_SYSCOUNTER` is active).

Signed-off-by: Adam Kondraciuk <adam.kondraciuk@nordicsemi.no>