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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/8250.txt

···
 - 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: