soc: mscc: ocelot: add MII registers description

Add the register definitions for the MSCC MIIM MDIO controller in
preparation for seville_vsc9959.c to create its accessors for the
internal MDIO bus.

Since we've introduced elements to ocelot_regfields that are not
instantiated by felix and ocelot, we need to define the size of the
regfields arrays explicitly, otherwise ocelot_regfields_init, which
iterates up to REGFIELD_MAX, will fault on the undefined regfield
entries (if we're lucky).

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>