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>