pwm-backlight: retrieve configured PWM period

The PWM core is now able to initialize the PWM period from a lookup
table defined by board files. Use it if available and fallback to the
value supplied in pwm_period_ns.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>