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 <sebastian.glab@nordicsemi.no>