leds: leds-pwm: convert OF parsing code to use led_pwm_add()

Convert the OF parsing code to use the common PWM LED registration code,
which means we have a consistent method, and single point where the
registration happens for both paths.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Bryan Wu <cooloney@gmail.com>