tests: drivers: timer: nrf_grtc_timer: add nRF54L15 FLPR to targets

Allow testing FLPR with this testcase.

Signed-off-by: Marcin Szymczyk <[email protected]>

Author: masz-nordic

tests/drivers/timer/nrf_grtc_timer/testcase.yaml

@@ -3,5 +3,6 @@ tests:
     tags: drivers
     platform_allow:
       - nrf54l15pdk/nrf54l15/cpuapp
+      - nrf54l15pdk/nrf54l15/cpuflpr
       - nrf54h20dk/nrf54h20/cpuapp
       - nrf54h20dk/nrf54h20/cpurad