tests: drivers: rtc: add rtc node for testing

Add rtc overlay for lp_mspm0g3507 to enable test coverage

Signed-off-by: Karthikeyan Krishnasamy <karthikeyan@linumiz.com>

Author: karthi012

tests/drivers/rtc/rtc_api/boards/lp_mspm0g3507.overlay

@@ -0,0 +1,15 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright (c) 2025 Linumiz GmbH
+ */
+
+/ {
+	aliases {
+		rtc = &rtc;
+	};
+};
+
+&rtc {
+	status = "okay";
+};