Documentation: dt: serial: Add TX FIFO threshold parameter

Add the device tree binding needed to support the TX FIFO threshold
parameter.

Signed-off-by: Thor Thayer <tthayer@opensource.altera.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/8250.txt

@@ -42,6 +42,8 @@ Optional properties:
 - auto-flow-control: one way to enable automatic flow control support. The
   driver is allowed to detect support for the capability even without this
   property.
+- tx-threshold: Specify the TX FIFO low water indication for parts with
+  programmable TX FIFO thresholds.
 
 Note:
 * fsl,ns16550: