drivers: pwm: nrf_sw: support generating PWM based on RTC

So far nRF's TIMER was used for generating PWM signal. Add support for
generating PWM based on RTC, which is sourced by 32KHz low frequency
crystal. This allows to use low frequency PWM with much lower power
consumption, because high frequency clock path can be disabled.

Don't support RTC clock prescaler, because maximum 512s period covers
most use cases. This allows to adjust pulse and period cycles to the
fact that CLEAR task event is generated always one LFCLK cycle after
period COMPARE value is reached.

Also update hal_nordic revision, as it contains updated check for PPI
channels conflict when RTC is used to generate PWM.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>