net: phy: dp83867: Add support for hardware blinking LEDs

This implements the led_hw_* hooks to support hardware blinking LEDs on
the DP83867 phy. The driver supports all LED modes that have a
corresponding TRIGGER_NETDEV_* define. Error and collision do not have
a TRIGGER_NETDEV_* define, so these modes are currently not supported.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com> #TQMa8MxML/MBa8Mx
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>