boards: arm: stm32: set LEDs period to 20ms

Most LEDs had 0 or 4 nanoseconds set as a period, a value that doesn't
make sense for a PWM signal driving an LED. A period of 20ms (50Hz) is a
frequently used value as it is above the flicker fusion threshold. All
STM32 based boards have been updated.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>