leds: leds-gpio: Make use of device property API

Make use of device property API in this driver so that both OF and ACPI
based system can use the same driver.

This change contains material from Max Eliaser and Mika Westerberg.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Bryan Wu <cooloney@gmail.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>