tests: drivers: rtc: add siwg917_rb4338a board overlay and add rtc tag

Add rtc tag to the siwg917_rb4338a board to activate testing on the
board.
Also add an overlay for the board in order to run the test correctly.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>

boards/silabs/radio_boards/siwx917_rb4338a/siwx917_rb4338a.yaml

@@ -18,4 +18,5 @@ supported:
   - watchdog
   - spi
   - wifi
+  - rtc
 vendor: silabs

tests/drivers/rtc/rtc_api/boards/siwx917_rb4338a.overlay

@@ -0,0 +1,15 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright (c) 2025 Silicon Laboratories Inc.
+ */
+
+/ {
+	aliases {
+		rtc = &rtc0;
+	};
+};
+
+&rtc0 {
+	status = "okay";
+};