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>