[nrf fromtree] tests: drivers: counter: counter_basic_api: Fix test on nrf54l15 NS

Add DTS overlay required to pass the test.
Test filter is meet for nrf54l15 NS platform.
However, test fails due to missing node configuration.

Fix the counter test on nrf54l15dk/nrf54l15/cpuapp/ns platform.

Signed-off-by: Sebastian Głąb <[email protected]>
(cherry picked from commit 89723c1)

Author: nordic-segl

tests/drivers/counter/counter_basic_api/boards/nrf54l15dk_nrf54l15_cpuapp_ns.overlay

@@ -0,0 +1,7 @@
+/*
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "nrf54l15dk_nrf54l15_common.dtsi"