net: mscc: ocelot: keep ocelot_stat_layout by reg address, not offset

With so many counter addresses recently discovered as being wrong, it is
desirable to at least have a central database of information, rather
than two: one through the SYS_COUNT_* registers (used for
ndo_get_stats64), and the other through the offset field of struct
ocelot_stat_layout elements (used for ethtool -S).

The strategy will be to keep the SYS_COUNT_* definitions as the single
source of truth, but for that we need to expand our current definitions
to cover all registers. Then we need to convert the ocelot region
creation logic, and stats worker, to the read semantics imposed by going
through SYS_COUNT_* absolute register addresses, rather than offsets
of 32-bit words relative to SYS_COUNT_RX_OCTETS (which should have been
SYS_CNT, by the way).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>