Add control of an external VSC7512 chip.

Currently the four copper phy ports are fully functional. Communication to
external phys is also functional, but the SGMII / QSGMII interfaces are
currently non-functional.
Signed-off-by: NColin Foster <colin.foster@in-advantage.com>
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Reviewed-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com> # regression
Signed-off-by: NJakub Kicinski <kuba@kernel.org>