machine: refactor PWM support

This commit lays the groundwork and refactors the SAM D21
implementation.

Author: aykevl

src/machine/machine.go

@@ -37,10 +37,6 @@ func (p Pin) Low() {
 	p.Set(false)
 }
 
-type PWM struct {
-	Pin Pin
-}
-
 type ADC struct {
 	Pin Pin
 }

src/machine/machine_atmega328p.go

@@ -21,6 +21,10 @@ func (p Pin) getPortMask() (*volatile.Register8, uint8) {
 	}
 }
 
+type PWM struct {
+	Pin Pin
+}
+
 // InitPWM initializes the registers needed for PWM.
 func InitPWM() {
 	// use waveform generation