leds: lp55xx: handle enable pin in driver

This patch moves the handling of the chip's enable pin from the board
code into the driver. It also updates all board-code files using the
driver to incorporate this change.

This is needed for device tree support of the enable pin.

Signed-off-by: Sebastian Reichel <sre@debian.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>