mdio: Add register definitions for MDIO (clause 45)

IEEE 802.3 clause 45 specifies the MDIO interface and registers for
use in 10G and other PHYs, similar to the MII management interface.

PHYs may have up to 32 MMDs corresponding to different sub-layers and
functions, each with up to 65536 registers.  These are addressed by
PRTAD (similar to the MII PHY address) and DEVAD.  Define a mapping
for specifying PRTAD and DEVAD through the existing MII ioctls.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>