[nrf fromlist] tests: drivers: timer: nrf_grtc: run GRTC tests on nRF54L20

Run GRTC tests for nRF54L20 platform.

Upstream PR: zephyrproject-rtos/zephyr#76431

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

Author: adamkondraciuk

tests/drivers/timer/nrf_grtc_timer/testcase.yaml

@@ -8,3 +8,4 @@ tests:
       - nrf54h20dk/nrf54h20/cpuapp
       - nrf54h20dk/nrf54h20/cpurad
       - nrf54h20dk/nrf54h20/cpuppr
+      - nrf54l20pdk/nrf54l20/cpuapp