leds-lp55xx: use lp55xx common led registration function

leds-lp55xx: use lp55xx common led registration function

 LED class devices are registered in lp5521_register_leds() and
 lp5523_register_leds().
 Two separate functions are merged into consolidated lp55xx function,
 lp55xx_register_leds().

 Error handling fix:
 Unregistering LEDS are handled in lp55xx_register_leds() when LED registration
 failure occurs. So each driver error handler is changed to 'err_register_leds'

 Chip dependency: 'brightness_work_fn' and 'set_led_current'
 To make the structure abstract, both functions are configured in each driver.
 Those functions should be done by each driver because register control is
 chip-dependant work.

 lp55xx_init_led: skeleton
 Will be filled in next patch

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