drivers: pwm: nrf_sw: convert to timer phandle instead instance number

So far 'timer-instance' DT property was the way to configure TIMER which
was used for generating PWM signal. Replace that by using 'generator'
phandle, so we get prepared for supporting RTC instances as PWM signal
generator.

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