[nrf fromtree] tests: drivers: uart: async_api: add stm32u3 boards overlays

Add UART test overlays for Nucleo U385RG-Q board.
Remove non serial boot conf file since they are now unnecessary.

Signed-off-by: Khaoula Bidani <[email protected]>
(cherry picked from commit 065900b)

Author: kbidani

tests/drivers/uart/uart_async_api/boards/nucleo_u385rg_q.overlay

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2025 STMicroelectronics
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+dut: &lpuart1 {
+	dmas = <&gpdma1 0 35 STM32_DMA_PERIPH_TX
+		&gpdma1 1 34 STM32_DMA_PERIPH_RX>;
+	dma-names = "tx", "rx";
+};
+
+&gpdma1 {
+	status = "okay";
+};