net: phy: micrel: move phy reg offsets to common header

Some micrel devices share the same PHY register defines. This patch
moves them to one common header so other drivers can reuse them.
And reuse generic MII_* defines where possible.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>