tests: drivers: uart_async_api: enable it for m33 of imx943_evk

This commit enables tests/driver/uart/async_api demo
for m33 in NETCMIX of imx943_evk.
- build command as follows,
  west build -p always -b imx943_evk/mimx94398/m33
  tests/drivers/uart/uart_async_api

Signed-off-by: Biwen Li <biwen.li@nxp.com>

Author: biwenli

tests/drivers/uart/uart_async_api/boards/imx943_evk_mimx94398_m33.conf

@@ -0,0 +1 @@
+CONFIG_TEST_USERSPACE=n

tests/drivers/uart/uart_async_api/boards/imx943_evk_mimx94398_m33.overlay

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+
+dut: &lpuart3 {
+	status = "okay";
+	current-speed = <115200>;
+	nxp,loopback;
+};