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_atmega1284p.go

@@ -14,12 +14,58 @@ func CPUFrequency() uint32 {
 	return 20000000
 }
 
+const (
+	portA Pin = iota * 8
+	portB
+	portC
+	portD
+)
+
+const (
+	PA0 = portA + 0
+	PA1 = portA + 1
+	PA2 = portA + 2
+	PA3 = portA + 3
+	PA4 = portA + 4
+	PA5 = portA + 5
+	PA6 = portA + 6
+	PA7 = portA + 7
+	PB0 = portB + 0
+	PB1 = portB + 1
+	PB2 = portB + 2
+	PB3 = portB + 3
+	PB4 = portB + 4
+	PB5 = portB + 5
+	PB6 = portB + 6
+	PB7 = portB + 7
+	PC0 = portC + 0
+	PC1 = portC + 1
+	PC2 = portC + 2
+	PC3 = portC + 3
+	PC4 = portC + 4
+	PC5 = portC + 5
+	PC6 = portC + 6
+	PC7 = portC + 7
+	PD0 = portD + 0
+	PD1 = portD + 1
+	PD2 = portD + 2
+	PD3 = portD + 3
+	PD4 = portD + 4
+	PD5 = portD + 5
+	PD6 = portD + 6
+	PD7 = portD + 7
+)
+
+// getPortMask returns the PORTx register and mask for the pin.
 func (p Pin) getPortMask() (*volatile.Register8, uint8) {
-	if p < 8 {
-		return avr.PORTD, 1 << uint8(p)
-	} else if p < 14 {
-		return avr.PORTB, 1 << uint8(p-8)
-	} else {
-		return avr.PORTC, 1 << uint8(p-14)
+	switch {
+	case p >= PA0 && p <= PA7:
+		return avr.PORTA, 1 << uint8(p-portA)
+	case p >= PB0 && p <= PB7:
+		return avr.PORTB, 1 << uint8(p-portB)
+	case p >= PC0 && p <= PC7:
+		return avr.PORTC, 1 << uint8(p-portC)
+	default:
+		return avr.PORTD, 1 << uint8(p-portD)
 	}
 }