leds: lp55xx: enable setting default trigger

This enables setting a default trigger on an LP55xx channel,
either from platform data or device tree. This mechanism is
identical to the mechanism for GPIO LEDs and references the
common LEDs device tree bindings.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Milo Kim <milo.kim@ti.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>