samples: drivers: counter: alarm: Cleanup platform_allow for nrf54*

Remove duplicated test configuration after
nrf54l20pdk/nrf54l20/cpuxxx was renamed to
nrf54lm20dk/nrf54lm20a/cpuxxx.

Extend platform_allow with other nrf54* targets (overlays for these
targets already exist in the boards directory).

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl

samples/drivers/counter/alarm/boards/nrf54l20pdk_nrf54l20_cpuapp.overlay

@@ -21,6 +21,13 @@ tests:
       - nrf51dk/nrf51822
       - nrf52dk/nrf52832
       - nrf52840dk/nrf52840
+      - nrf54h20dk/nrf54h20/cpuapp
+      - nrf54h20dk/nrf54h20/cpuflpr
+      - nrf54h20dk/nrf54h20/cpuppr
+      - nrf54h20dk/nrf54h20/cpurad
+      - nrf54l15dk/nrf54l15/cpuapp
+      - nrf54l15dk/nrf54l15/cpuflpr
+      - nrf54lm20dk/nrf54lm20a/cpuapp
       - nrf9160dk/nrf9160
       - samd20_xpro
       - bl5340_dvk/nrf5340/cpuapp