leds: Introduce devres helper for led_classdev_register

(cooloney@gmail.com: add _unregister function into the document)

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>