leds: Add backlight LED trigger

This allows LEDs to be controlled as a backlight device where
they turn off and on when the display is blanked and unblanked.
This is useful where you need various key backlight LEDs to
dim at the same time as the backlight.

Signed-off-by: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>