leds: leds-pwm: Convert to use devm_get_pwm

Update the driver to use the new API for requesting pwm so we can take
advantage of the pwm_lookup table to find the correct pwm to be used for the
LED functionality.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>