PWM interface not consistent across implementations

[cibomahto]

The PWM interface appears to be only partially consistent across the various machine targets that implement it. I did a survey of the public interfaces for each of the implementations:

tinygo pwm spelunking.ods

Across implementations, there are some inconsistencies:

• The name of the PWM object type is not consistent across implementations. It is variously: timerType, PWM, pwmGroup, TCC, TIM
• The name of the exported PWM objects is not consistent. It is variously: Timerx, PWMx, TCCx, TIMx

And in specific implementations, there are some issues as well:

• machine_generic.go: Missing SetPeriod(), Period(), Counter(), and SetInverting(). Should probably be added since they seem to be part of the 'standard' interface
• machine_rp2.go: The PWM type is not exported, so references to the exported PWM objects cannot be stored in, for example, a project pin initialization list, without extra work
• machine_atsamd21.go, machine_atsamd51.go: Missing Period()
• machine_esp32xx_pwm.go: Missing SetPeriod(), Period(), and Counter()
• machine_nrf52xxx.go: Missing Period() and Counter()
• machine_stm32_tim.go: Missing Period(). Counter() is mis-named Count()

It's also not clear to me that there is a written standard for the PWM interface. I attempted to document it here: tinygo-org/tinygo-site#505

Assuming that it is desirable to regularize these, I would be happy to help out with any of the platforms that I have dev hardware for (I think everything except atsamdxx).

[deadprogram]

@cibomahto you are correct that things are non-consistent. This bit me recently, so I am quite in favor of some standardization.

Comments from other people please!