net: mscc: ocelot: export ethtool MAC Merge stats for Felix VSC9959

The Felix VSC9959 switch supports frame preemption and has a MAC Merge
layer. In addition to the structured stats that exist for the eMAC,
export the counters associated with its pMAC (pause, RMON, MAC, PHY,
control) plus the high-level MAC Merge layer stats. The unstructured
ethtool counters, as well as the rtnl_link_stats64 were left to report
only the eMAC counters.

Because statistics processing is quite self-contained in ocelot_stats.c
now, I've opted for introducing an ocelot->mm_supported bool, based on
which the common switch lib does everything, rather than pushing the
TSN-specific code in felix_vsc9959.c, as happens for other TSN stuff.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>