net: fec: add C45 MDIO read/write support

IEEE 802.3ae clause 45 defines a modified MDIO protocol that uses a two
staged access model in order to increase the address space.

This patch adds support for C45 MDIO read and write accesses, which are
used whenever the MII_ADDR_C45 flag in the regnum argument is set.
In case it is not set, C22 accesses are used as before.

Signed-off-by: Marco Hartmann <marco.hartmann@nxp.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>