machine/atsamd21: refactor SPI pin handling to only look at pin numbers

Pin mode and pad numbers are automatically calculated from the pin
numbers, returning an error if no pinout could be found.

src/machine/machine.go

@@ -1,5 +1,12 @@
 package machine
 
+import "errors"
+
+var (
+	ErrInvalidInputPin  = errors.New("machine: invalid input pin")
+	ErrInvalidOutputPin = errors.New("machine: invalid output pin")
+)
+
 type PinConfig struct {
 	Mode PinMode
 }