machine/samd21: use pins specified in I2CConfig

Instead of configuring machine.I2C0, machine.I2C1, etc. statically,
allow the pins to be set using machine.I2CConfig. This will also
automatically configure the correct pin mode for each pin instead of
having to specify that manually.

src/machine/machine.go

@@ -5,6 +5,8 @@ import "errors"
 var (
 	ErrInvalidInputPin  = errors.New("machine: invalid input pin")
 	ErrInvalidOutputPin = errors.New("machine: invalid output pin")
+	ErrInvalidClockPin  = errors.New("machine: invalid clock pin")
+	ErrInvalidDataPin   = errors.New("machine: invalid data pin")
 )
 
 type PinConfig struct {