leds: pwm: Use OF variant of LED registering function

The PWM leds can be instantiated from Device Tree so pass the
respective device node to LED core.  This provides the LED system with
proper device node and exposes it through uevent.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>