net: phy: Add mdio-aspeed

The AST2600 design separates the MDIO controllers from the MAC, which is
where they were placed in the AST2400 and AST2500. Further, the register
interface is reworked again, so now we have three possible different
interface implementations, however this driver only supports the
interface provided by the AST2600. The AST2400 and AST2500 will continue
to be supported by the MDIO support embedded in the FTGMAC100 driver.

The hardware supports both C22 and C45 mode, but for the moment only C22
support is implemented.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>