tty: ar933x_uart: allow to build the driver as a module

Change the SERIAL_AR933X Kconfig option from
'bool' to 'tristate' in order to allow to build
the driver as a module. Also extend the help text
of the option to reflect that.
Signed-off-by: NGabor Juhos <juhosg@openwrt.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -1401,13 +1401,16 @@ config SERIAL_XILINX_PS_UART_CONSOLE
 	  Enable a Xilinx PS UART port to be the system console.
 
 config SERIAL_AR933X
-	bool "AR933X serial port support"
+	tristate "AR933X serial port support"
 	depends on HAVE_CLK && SOC_AR933X
 	select SERIAL_CORE
 	help
 	  If you have an Atheros AR933X SOC based board and want to use the
 	  built-in UART of the SoC, say Y to this option.
 
+	  To compile this driver as a module, choose M here: the
+	  module will be called ar933x_uart.
+
 config SERIAL_AR933X_CONSOLE
 	bool "Console on AR933X serial port"
 	depends on SERIAL_AR933X=y