boards: arm: twr_ke18f: fix PWM LEDs period

The period was set to 60 usec, a value that doesn't make much sense in
the context of PWM driven LEDs. Since the period cell was rarely used,
the value was likely added because it is required, but was never used. A
period of 20 msec has been chosen as most other boards do.

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