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_fe310.go

@@ -44,7 +44,8 @@ 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 the pin is configured as an
+// input or as an output.
 func (p Pin) Get() bool {
 	val := sifive.GPIO0.VALUE.Get() & (1 << uint8(p))
 	return (val > 0)