ptp: track available ptp vclocks information

Track available ptp vclocks information. Record index values
of available ptp vclocks during registering and unregistering.

This is preparation for supporting ptp vclocks info query
through ethtool.

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