Document new Hardware driven LEDs APIs.

Some LEDs can be programmed to be driven by hardware. This is not
limited to blink but also to turn off or on autonomously.
To support this feature, a LED needs to implement various additional
ops and needs to declare specific support for the supported triggers.

Add documentation for each required value and API to make hw control
possible and implementable by both LEDs and triggers.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>