ARM: pxa: hx4700: initialize all the struct pwm_lookup members

Instead of relying on the .pwm_period_ns member of the pwm-backlight
driver's platform data, the PWM period can be retrieved from the PWM
lookup table.

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