dt-bindings: ptp: add ptp-qoriq.txt

This patch is to add a documentation for ptp_qoriq dt-bindings.
The description for ptp_qoriq dt-bindings was actually moved
from Documentation/devicetree/bindings/net/fsl-tsec-phy.txt,
since gianfar_ptp driver was moved to ptp_qoriq driver.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>