leds: lp55xx: add DT bindings for LP55231

The TI55231 appears to be fully compatible to the 5523 model from
National Semicondutor. This patch just adds DT bindings for it.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>