net: mscc: ocelot: expose regfield definition to be used by other drivers

The ocelot_regfields struct is common between several different chips, some
of which can only be controlled externally. Export this structure so it
doesn't have to be duplicated in these other drivers.

Rename the structure as well, to follow the conventions of other shared
resources.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com> # regression
Signed-off-by: Jakub Kicinski <kuba@kernel.org>