unicore32: nb0916: Use PWM lookup table

Use a PWM lookup table to provide the PWM to the pwm-backlight device.
The driver has a legacy code path that is required only because boards
still use the legacy method of requesting PWMs by global ID. Replacing
these usages allows that legacy fallback to be removed.

Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>