net: phy: Expose phydev::dev_flags through sysfs

phydev::dev_flags contains a bitmask of configuration bits requested by
the consumer of a PHY device (Ethernet MAC or switch) towards the PHY
driver. Since these flags are often used for requesting LED or other
type of configuration being able to quickly audit them without
instrumenting the kernel is useful.

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