Documentation: leds: Update 00-INDEX file

Add missing entries for the following documentation files:

- leds-class-flash.txt
- leds-lm3556.txt
- leds-mlxcpld.txt
- ledtrig-usbport.txt
- uleds.txt

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>