dt-binding: ptp_qoriq: document "fsl,extts-fifo" property

Documented "fsl,extts-fifo" property.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>