boards: nrf: Fix sw_pwm channel definitions

This is a follow-up to commit f301dc23.

Add missing GPIO_ACTIVE_LOW flag to sw_pwm channel definitions for
nRF51 DK and nRF51 Dongle so that the initial state of their PWM
outputs is properly set up.
Add missing sw_pwm channel definition in the nrf52833dk_nrf52820 target
that was modified in 793362ae instead
of in f301dc23. Add also the pwm-led0
alias for its PWM LED so that it can be used with standard samples.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>