avr: unify GPIO pin/port code

All the AVRs that I've looked at had the same pin/port structure, with
the possible states being input/floating, input/pullup, low, and high
(with the same PORT/DDR registers). The main difference is the number of
available ports and pins. To reduce the amount of code and avoid
duplication (and thus errors) I decided to centralize this, following
the design used by the atmega2560 but while using a trick to save
tracking a few registers.

In the process, I noticed that the Pin.Get() function was incorrect on
the atmega2560 implementation. It is now fixed in the unified code.

src/machine/machine_attiny85.go

@@ -16,21 +16,8 @@ const (
 	PB5
 )
 
-// Configure sets the pin to input or output.
-func (p Pin) Configure(config PinConfig) {
-	if config.Mode == PinOutput { // set output bit
-		avr.DDRB.SetBits(1 << uint8(p))
-	} else { // configure input: clear output bit
-		avr.DDRB.ClearBits(1 << uint8(p))
-	}
-}
-
+// getPortMask returns the PORTx register and mask for the pin.
 func (p Pin) getPortMask() (*volatile.Register8, uint8) {
+	// Very simple for the attiny85, which only has a single port.
 	return avr.PORTB, 1 << uint8(p)
 }
-
-// Get returns the current value of a GPIO pin.
-func (p Pin) Get() bool {
-	val := avr.PINB.Get() & (1 << uint8(p))
-	return (val > 0)
-}