ptp: add kernel API ptp_get_vclocks_index()

Add kernel API ptp_get_vclocks_index() to get all ptp
vclocks index on pclock.

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>