leds: lp8860: Use generic support for composing LED names

Switch to using generic LED support for composing LED class
device name.

While at it, avoid iterating through available child of nodes
in favor of obtaining single expected child node using single
call to of_get_next_available_child().

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Dan Murphy <dmurphy@ti.com>