drivers: serial: Add DTC support for serial driver and implement Async API

Update serial driver support for RX MCU:
- Add DTC support for SCI UART driver.
- Implementation Async APIs for serial driver.

Signed-off-by: Phi Tran <phi.tran.jg@bp.renesas.com>

Author: Phi Tran

drivers/serial/Kconfig.renesas_rx

@@ -1,4 +1,4 @@
-# Copyright (c) 2024 Renesas Electronics Corporation
+# Copyright (c) 2024-2025 Renesas Electronics Corporation
 # SPDX-License-Identifier: Apache-2.0
 
 config UART_RENESAS_RX
@@ -7,7 +7,9 @@ config UART_RENESAS_RX
 	depends on DT_HAS_RENESAS_RX_UART_SCI_ENABLED
 	select SERIAL_HAS_DRIVER
 	select SERIAL_SUPPORT_INTERRUPT
+	select SERIAL_SUPPORT_ASYNC
 	select USE_RX_RDP_SCI_UART
+	select RENESAS_RX_DTC if UART_ASYNC_API
 	select PINCTRL
 	help
 	  Enable Renesas RX series UART driver.