boards: arm: legend: fix PWM LEDs period

The period was 255 nsec, a value that doesn't make much sense when
driving an LED. Since the period cell is rarely used nowadays, the
value was probably copy&pasted or a random value was added since it is
required. A period of 20 msec has been chosen as most other boards do
for PWM LEDs.

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