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: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>