net: mscc: ocelot: add ops for decoding watermark threshold and occupancy

We'll need to read back the watermark thresholds and occupancy from
hardware (for devlink-sb integration), not only to write them as we did
so far in ocelot_port_set_maxlen. So introduce 2 new functions in struct
ocelot_ops, similar to wm_enc, and implement them for the 3 supported
mscc_ocelot switches.

Remove the INUSE and MAXUSE unpacking helpers for the QSYS_RES_STAT
register, because that doesn't scale with the number of switches that
mscc_ocelot supports now. They have different bit widths for the
watermarks, and we need function pointers to abstract that difference
away.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>