[nrf fromtree] tests: drivers: uart: uart_async_api: Add nrf54l15dk//cpuapp overlay

Add overlay for nrf54l15dk/nrf54l15/cpuapp target.

Signed-off-by: Krzysztof Chruściński <[email protected]>
(cherry picked from commit a04c1db)

Author: nordic-krch

tests/drivers/uart/uart_async_api/boards/nrf54l15dk_nrf54l15_cpuapp.overlay

@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+
+&pinctrl {
+	uart21_default_alt: uart21_default_alt {
+		group1 {
+			psels = <NRF_PSEL(UART_TX, 1, 10)>,
+				<NRF_PSEL(UART_RX, 1, 11)>;
+		};
+	};
+
+	uart21_sleep_alt: uart21_sleep_alt {
+		group1 {
+			psels = <NRF_PSEL(UART_TX, 1, 10)>,
+				<NRF_PSEL(UART_RX, 1, 11)>;
+			low-power-enable;
+		};
+	};
+};
+
+dut: &uart21 {
+	status = "okay";
+	pinctrl-0 = <&uart21_default_alt>;
+	pinctrl-1 = <&uart21_sleep_alt>;
+	pinctrl-names = "default", "sleep";
+	current-speed = <115200>;
+};