leds-lp55xx: clean up init leds in lp5521/5523

 To make LED initialization code simple, new function, _register_leds()
 is added at each driver.
 This patch is a preceding step for lp55xx common driver architecture.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>