ARM: OMAP3: Beagle: initialize all the struct pwm_lookup members

The PWM core can retrieve the period from the PWM lookup table, so the
struct led_pwm.pwm_period_ns member can be removed.

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