mlxsw: spectrum_ptp: Rename mlxsw_sp1_ptp_phc_adjfreq()

The function mlxsw_sp_ptp_phc_adjfreq() configures MTUTC register to adjust
hardware frequency by a given value.

This configuration will be same for Spectrum-2. In preparation for
Spectrum-2 PTP support, rename the function to not be Spectrum-1 specific.
Later, it will be used for Spectrum-2 also.

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>