Add support for additional PWM channels

[golemparts]

Devices with an RP1 or equivalent (Pi 5 and later) are able to support up to 4 PWM channels at the same time on any GPIO (see the pwm-pio overlay). Additionally, software PWM using pwm-gpio can also configure any GPIO as a PWM channel. Neither option is currently supported by RPPAL, and needs additional research (which pwmchip is used for either option?) and an eventual implementation.

It may also be useful to add an alternative Pwm constructor that accepts a complete sysfs path for situations where it's impossible to automatically determine the correct path.

[golemparts]

Addressed in 0.22.0