net: phy: leds: Add support for "link" trigger

Currently, we create a LED trigger for any link speed known to a PHY.
These triggers only fire when their exact link speed had been negotiated
(they aren't cumulative, that is, they don't fire for "their or any higher"
link speed).

What we are missing, however, is a trigger which will fire on any link
speed known to the PHY. Such trigger can then be used for implementing a
poor man's substitute of the "link" LED on boards that lack it.
Let's add it.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: David S. Miller <davem@davemloft.net>