machine: support Pin.Get() function when the pin is configured as output

To my surprise, this is supported on all the devices I could test so
therefore it makes sense to change the API to allow this.

src/machine/machine_atsamd21g18.go

@@ -82,7 +82,8 @@ func (p Pin) Set(high bool) {
 	}
 }
 
-// Get returns the current value of a GPIO pin.
+// Get returns the current value of a GPIO pin when configured as an input or as
+// an output.
 func (p Pin) Get() bool {
 	if p < 32 {
 		return (sam.PORT.IN0.Get()>>uint8(p))&1 > 0