net: sysfs: Document PHY device sysfs attributes

Document the different sysfs attributes that exist for PHY devices:
attached_dev, phy_has_fixups, phy_id and phy_interface.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>