tests: drivers: rtc: Add rtc_api overlay and update yaml file

This commit adds the rtc_api overlay file for the brd4342a board,
enabling rtc support and also updates the rtc tag in
siwx917_rb4342a.yaml.

Upstream-status: available
Signed-off-by: S Mohamed Fiaz <[email protected]>
(cherry picked from commit 8dda8fc)

Author: fimohame

boards/silabs/radio_boards/siwx917_rb4342a/siwx917_rb4342a.yaml

@@ -20,6 +20,7 @@ supported:
   - pwm
   - uart
   - watchdog
+  - rtc
   - counter
   - wifi
 vendor: silabs

tests/drivers/rtc/rtc_api/boards/siwx917_rb4342a.overlay

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