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 <thomas.petazzoni@free-electrons.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/phy.h

@@ -75,6 +75,7 @@ typedef enum {
 	PHY_INTERFACE_MODE_SMII,
 	PHY_INTERFACE_MODE_XGMII,
 	PHY_INTERFACE_MODE_MOCA,
+	PHY_INTERFACE_MODE_QSGMII,
 	PHY_INTERFACE_MODE_MAX,
 } phy_interface_t;
 
@@ -116,6 +117,8 @@ static inline const char *phy_modes(phy_interface_t interface)
 		return "xgmii";
 	case PHY_INTERFACE_MODE_MOCA:
 		return "moca";
+	case PHY_INTERFACE_MODE_QSGMII:
+		return "qsgmii";
 	default:
 		return "unknown";
 	}