phylib: mdio: Add sysfs attribute for PHY identifiers.

This adds a phy_id sysfs attribute to MDIO devices, containing the
32-bit PHY identifier reported by the device.  This attribute can
be useful when debugging problems related to phy drivers.  Other
enumerable buses already have similar attributes.
Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/ABI/testing/sysfs-bus-mdio

+What:		/sys/bus/mdio_bus/devices/.../phy_id
+Date:		November 2012
+KernelVersion:	3.8
+Contact:	netdev@vger.kernel.org
+Description:
+		This attribute contains the 32-bit PHY Identifier as reported
+		by the device during bus enumeration, encoded in hexadecimal.
+		This ID is used to match the device with the appropriate
+		driver.

drivers/net/phy/mdio_bus.c

@@ -431,10 +431,24 @@ static struct dev_pm_ops mdio_bus_pm_ops = {
 
 #endif /* CONFIG_PM */
 
+static ssize_t
+phy_id_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct phy_device *phydev = to_phy_device(dev);
+
+	return sprintf(buf, "0x%.8lx\n", (unsigned long)phydev->phy_id);
+}
+
+static struct device_attribute mdio_dev_attrs[] = {
+	__ATTR_RO(phy_id),
+	__ATTR_NULL
+};
+
 struct bus_type mdio_bus_type = {
 	.name		= "mdio_bus",
 	.match		= mdio_bus_match,
 	.pm		= MDIO_BUS_PM_OPS,
+	.dev_attrs	= mdio_dev_attrs,
 };
 EXPORT_SYMBOL(mdio_bus_type);