mlxsw: reg: Add MTUTC register's fields for supporting PTP in Spectrum-2

The MTUTC register configures the HW UTC counter.

Add the relevant fields and operations to support PTP in Spectrum-2 and
update mlxsw_reg_mtutc_pack() with the new fields for a future use.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>