net: phy: mscc: add support for VSC8502

This is a dual copper PHY with support for MII/GMII/RGMII on MAC side,
as well as a bunch of other features such as SyncE and Ring Resiliency.

I haven't tested interrupts and WoL, but I am confident that they work
since support is already present in the driver and the register map is
no different for this PHY.

PHY statistics work, PHY tunables appear to work, suspend/resume works.

Signed-off-by: Wes Li <wes.li@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>