net: dsa: mv88e6xxx: add support for event capture

This patch adds support for configuring mv88e6xxx GPIO lines as PTP
pins, so that they may be used for time stamping external events or for
periodic output.

Checkpatch and reverse Christmas tree fixes by Andrew Lunn

Periodic output removed by Richard Cochran, until a better abstraction
of a VCO is added to Linux in general.

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