Documentation: devicetree: add PHY lane swap binding

Add the documentation for PHY lane swapping.  This is a boolean entry to
notify the phy device drivers that the TX/RX lanes need to be swapped.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>