tests: drivers: uart: Add support test UART for ek_ra4c1

Add support test app ``uart_async_api`` for Renesas ek_ra4c1 board

Signed-off-by: Khoa Nguyen <[email protected]>

Author: khoa-nguyen-18

tests/drivers/uart/uart_async_api/boards/ek_ra4c1.overlay

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2025 Renesas Electronics Corporation
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&pinctrl {
+	sci9_default: sci9_default {
+		group1 {
+			/* tx rx */
+			psels = <RA_PSEL(RA_PSEL_SCI_9, 1, 9)>,
+			<RA_PSEL(RA_PSEL_SCI_9, 1, 10)>;
+		};
+	};
+};
+
+&sci9 {
+	pinctrl-0 = <&sci9_default>;
+	pinctrl-names = "default";
+	interrupts = <60 1>, <61 1>, <62 1>, <63 1>;
+	interrupt-names = "rxi", "txi", "tei", "eri";
+	status = "okay";
+
+	dut: uart {
+		current-speed = <115200>;
+		status = "okay";
+	};
+};