dt-bindings: net: ethernet-controller: Document support for LEDs node

Document support for LEDs node in ethernet-controller.
Ethernet Controller may support different LEDs that can be configured
for different operation like blinking on traffic event or port link.

Also add some Documentation to describe the difference of these nodes
compared to PHY LEDs, since ethernet-controller LEDs are controllable
by the ethernet controller regs and the possible intergated PHY doesn't
have control on them.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>