drivers: uart_mcux: Do not select async support

Commit f1b0b45 mistakenly selected the
SERIAL_SUPPORT_ASYNC kconfig for the kinetis uart
driver when enabling the lpuart driver. Revert this,
because the kinetis uart driver does not support async api.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>

Author: decsny

drivers/serial/Kconfig.mcux

@@ -10,7 +10,6 @@ config UART_MCUX
 	depends on CLOCK_CONTROL
 	select SERIAL_HAS_DRIVER
 	select SERIAL_SUPPORT_INTERRUPT
-	select SERIAL_SUPPORT_ASYNC
 	select PINCTRL
 	help
 	  Enable the MCUX uart driver.