net: pcs: xpcs: convert to mdio_device

Unify the 2 existing PCS drivers (lynx and xpcs) by doing a similar
thing on probe, which is to have a *_create function that takes a
struct mdio_device * given by the caller, and builds a private PCS
structure around that.

This changes stmmac to hold only a pointer to the xpcs, as opposed to
the full structure. This will be used in the next patch when struct
mdio_xpcs_ops is removed. Currently a pointer to struct mdio_xpcs_ops
is used as a shorthand to determine whether the port has an XPCS or not.
We can do the same now with the mdio_xpcs_args pointer.

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