leds-lp55xx: add new function for removing device attribtues

 lp55xx_unregister_sysfs() is used for removing lp55xx device attributes.
 Chip specific and engine attributes are removed on unloading the driver.

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