Add support for the DP83811 phy.

The DP83811 supports both rgmii and sgmii interfaces.
There are 2 part numbers for this the DP83TC811R does not
reliably support the SGMII interface but the DP83TC811S will.

There is not a way to differentiate these parts from the
hardware or register set.  So this is controlled via the DT
to indicate which phy mode is required.  Or the part can be
strapped to a certain interface.

Data sheet can be found here:
http://www.ti.com/product/DP83TC811S-Q1/description
http://www.ti.com/product/DP83TC811R-Q1/descriptionSigned-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>