net: phy: leds: add support for led triggers on phy link state change

Create an option CONFIG_LED_TRIGGER_PHY (default n), which will create a
set of led triggers for each instantiated PHY device. There is one LED
trigger per link-speed, per-phy.
The triggers are registered during phy_attach and unregistered during
phy_detach.

This allows for a user to configure their system to allow a set of LEDs
not controlled by the phy to represent link state changes on the phy.
LEDS controlled by the phy are unaffected.

For example, we have a board where some of the leds in the
RJ45 socket are controlled by the phy, but others are not. Using the
triggers provided by this patch the leds not controlled by the phy can
be configured to show the current speed of the ethernet connection. The
leds controlled by the phy are unaffected.

Signed-off-by: Josh Cartwright <josh.cartwright@ni.com>
Signed-off-by: Nathan Sullivan <nathan.sullivan@ni.com>
Signed-off-by: Zach Brown <zach.brown@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>