mlxsw: spectrum: PTP: Support ethtool get_ts_info

The get_ts_info callback is used for obtaining information about
timestamping capabilities of a network device. On Spectrum-1, implement
it to advertise the PHC and the capability to do HW timestamping, and
the supported RX and TX filters.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>