MAINTAINERS: add maintainer for LED subsystem

Add Bryan Wu as the primary maintainer for drivers/leds

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>