[nrf fromtree] tests: drivers: counter: counter_nrf_rtc: Run test on nrf54h20

Add nrf54h20dk to platform allow list.
Overlay for tht target was already added.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>
(cherry picked from commit 89489d6)

Author: nordic-segl

tests/drivers/counter/counter_nrf_rtc/fixed_top/testcase.yaml

@@ -7,3 +7,5 @@ tests:
     platform_allow:
       - nrf52840dk/nrf52840
       - nrf52_bsim
+      - nrf54h20dk/nrf54h20/cpuapp
+      - nrf54h20dk/nrf54h20/cpurad