drivers: timer: nrf_grtc_timer: add last_count initialization

The GRTC counter is not cleared at startup, therefore the
`last_count` variable needs to be initialized accordingly.
This change:
- Prevents overflow of the `sys_clock_announce()` int32_t parameter
- Ensures the correct uptime value, which should be reset during
  initialization

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