net: phy: add minimal support for QSGMII PHY

This commit adds the necessary definitions for the PHY layer to
recognize "qsgmii" as a valid PHY interface. A QSMII interface, as
defined at
http://en.wikipedia.org/wiki/Media_Independent_Interface#Quad_Serial_Gigabit_Media_Independent_Interface,
is "is a method of combining four SGMII lines into a 5Gbit/s
interface. QSGMII, like SGMII, uses LVDS signalling for the TX and RX
data and a single LVDS clock signal. QSGMII uses significantly fewer
signal lines than four SGMII busses."

This type of MAC <-> PHY connection might require special handling on
the MAC driver side, so it should be possible to express this type of
MAC <-> PHY connection, for example in the Device Tree.

Signed-off-by: Thomas Petazzoni <[e-mail]>
Cc: [e-mail]
Reviewed-by: Florian Fainelli <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/devicetree/bindings/net/ethernet.txt

···
 - max-frame-size: number, maximum transfer unit (IEEE defined MTU), rather than
   the maximum frame size (there's contradiction in ePAPR).
 - phy-mode: string, operation mode of the PHY interface; supported values are
-  "mii", "gmii", "sgmii", "tbi", "rev-mii", "rmii", "rgmii", "rgmii-id",
+  "mii", "gmii", "sgmii", "qsgmii", "tbi", "rev-mii", "rmii", "rgmii", "rgmii-id",
   "rgmii-rxid", "rgmii-txid", "rtbi", "smii", "xgmii"; this is now a de-facto
   standard property;
 - phy-connection-type: the same as "phy-mode" property but described in ePAPR;

include/linux/phy.h

···
 	PHY_INTERFACE_MODE_SMII,
 	PHY_INTERFACE_MODE_XGMII,
 	PHY_INTERFACE_MODE_MOCA,
+	PHY_INTERFACE_MODE_QSGMII,
 	PHY_INTERFACE_MODE_MAX,
 } phy_interface_t;
 
···
 		return "xgmii";
 	case PHY_INTERFACE_MODE_MOCA:
 		return "moca";
+	case PHY_INTERFACE_MODE_QSGMII:
+		return "qsgmii";
 	default:
 		return "unknown";
 	}