dt-bindings: ptp: Introduce MII time stamping devices.

This patch add a new binding that allows non-PHY MII time stamping
devices to find their buses.  The new documentation covers both the
generic binding and one upcoming user.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>