drivers: serial: ns16550: Add support for Async APIs

Added support for async APIs for ns16550. This will be
enabled by kconfig CONFIG_UART_ASYNC_API.

Signed-off-by: Anisetti Avinash Krishna <anisetti.avinash.krishna@intel.com>

Author: akanisetti

drivers/serial/Kconfig.ns16550

@@ -6,6 +6,7 @@ menuconfig UART_NS16550
 	depends on DT_HAS_NS16550_ENABLED
 	select SERIAL_HAS_DRIVER
 	select SERIAL_SUPPORT_INTERRUPT
+	select SERIAL_SUPPORT_ASYNC
 	help
 	  This option enables the NS16550 serial driver.
 	  This driver can be used for the serial hardware