net: mscc: ocelot: add MAC Merge layer support for VSC9959

Felix (VSC9959) has a DEV_GMII:MM_CONFIG block composed of 2 registers
(ENABLE_CONFIG and VERIF_CONFIG). Because the MAC Merge statistics and
pMAC statistics are already in the Ocelot switch lib even if just Felix
supports them, I'm adding support for the whole MAC Merge layer in the
common Ocelot library too.

There is an interrupt (shared with the PTP interrupt) which signals
changes to the MM verification state. This is done because the
preemptible traffic classes should be committed to hardware only once
the verification procedure has declared the link partner of being
capable of receiving preemptible frames.

We implement ethtool getters and setters for the MAC Merge layer state.
The "TX enabled" and "verify status" are taken from the IRQ handler,
using a mutex to ensure serialized access.

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