phylink: add support for MII ioctl access to Clause 45 PHYs

Add support for reading and writing the clause 45 MII registers.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>