[nrf fromlist] 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

Upstream PR #: 91432

Signed-off-by: Adam Kondraciuk <[email protected]>

Author: adamkondraciuk

drivers/timer/nrf_grtc_timer.c

@@ -485,6 +485,7 @@ static int sys_clock_driver_init(void)
 	}
 #endif /* CONFIG_NRF_GRTC_START_SYSCOUNTER */
 
+	last_count = counter();
 	int_mask = NRFX_GRTC_CONFIG_ALLOWED_CC_CHANNELS_MASK;
 	if (!IS_ENABLED(CONFIG_TICKLESS_KERNEL)) {
 		system_timeout_set_relative(CYC_PER_TICK);