Documentation: consolidate leds files to leds/ subdir

leds: move leds-class documentation under the leds/ subdir.
Add also a leds/00-INDEX file describing the files under leds/

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Acked-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>