machine: move PinMode to central location

It is always implemented exactly the same way (as an uint8) so there is
no reason to implement it in each target separately.

This also makes it easier to add some documentation to it.

src/machine/machine.go

@@ -10,6 +10,11 @@ var (
 	ErrNoPinChangeChannel = errors.New("machine: no channel available for pin interrupt")
 )
 
+// PinMode sets the direction and pull mode of the pin. For example, PinOutput
+// sets the pin as an output and PinInputPullup sets the pin as an input with a
+// pull-up.
+type PinMode uint8
+
 type PinConfig struct {
 	Mode PinMode
 }