mlxsw: spectrum: PTP: Add PTP initialization / finalization

Add two ptp_ops: init and fini, to initialize and finalize the PTP
subsystem. Call as appropriate from mlxsw_sp_init() and _fini().

Lay the groundwork for Spectrum-1 support. On Spectrum-1, the received
timestamped packets and their corresponding timestamps arrive
independently, and need to be matched up. Introduce the related data types
and add to struct mlxsw_sp_ptp_state the hash table that will keep the
unmatched entries.

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>