doc: dt-binding: of-serial: add Freescale 64-byte FIFO mode uart binding

This add the 64-byte FIFO mode device tree binding for Freescale DUART.
Signed-off-by: Jingchang Lu <jingchang.lu@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/of-serial.txt

@@ -14,6 +14,7 @@ Required properties:
 	- "altr,16550-FIFO32"
 	- "altr,16550-FIFO64"
 	- "altr,16550-FIFO128"
+	- "fsl,16550-FIFO64"
 	- "serial" if the port type is unknown.
 - reg : offset and length of the register set for the device.
 - interrupts : should contain uart interrupt.