drivers: pwm: nrf_sw: always default to yes

There's no need to add conditions to CONFIG_PWM_NRF_SW, because it is
already conditioned to DT status. Right now, if any system uses both
normal PWM and "SW PWM", it needs to manually enable the driver
Kconfig.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>