leds: pca963x: Make use of device property API

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

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>