[pwm] Convert to ipgen because number of outputs not configurable

[Razer6]

The PWM supports changing the number of outputs via a parameter defined in pwm.hjson. However, changing this parameter is not enough as the reg_top defines on the number of registers. In particular, there are multi-registers, that are instantiated for all outputs.

To tolve this problem and to make the PWM IP parametrizable, it needs to be converted to an ipgen IP.

/cc @alees24