tty: ar933x_uart: add device tree support and binding documentation

Modify the probe routine to get the port line number
from device tree if the 'of_node' is populated in the
platform device. The driver can be built as module,
thus add an OF specific module device table as well
to support module auto loading.

This makes it possible to use the driver for AR9330
UART devices specified in device tree.

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