tests: drivers: uart: async_dual: Update configuration for nrf54l

Add timer property to the uart device under test.
Add zephyr,pm-device-runtime-auto to the uart device under test.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

Author: nordic-krch

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

@@ -42,6 +42,7 @@ dut: &uart21 {
 	pinctrl-1 = <&uart21_sleep_alt>;
 	pinctrl-names = "default", "sleep";
 	current-speed = <115200>;
+	timer = <&timer21>;
 };
 
 dut2: &uart00 {

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

@@ -38,13 +38,19 @@
 	};
 };
 
+&timer21 {
+	status = "reserved";
+};
+
 dut: &uart21 {
 	status = "okay";
 	current-speed = <115200>;
 	pinctrl-0 = <&uart21_default>;
 	pinctrl-1 = <&uart21_sleep>;
 	pinctrl-names = "default", "sleep";
 	hw-flow-control;
+	timer = <&timer21>;
+	zephyr,pm-device-runtime-auto;
 };
 
 dut_aux: &uart22 {
@@ -54,6 +60,7 @@ dut_aux: &uart22 {
 	pinctrl-1 = <&uart22_sleep>;
 	pinctrl-names = "default", "sleep";
 	hw-flow-control;
+	zephyr,pm-device-runtime-auto;
 };
 
 &timer20 {