avr: use standard pin numbering

This commit changes pin numbering for atmega328 based boards (Uno, Nano)
to use the standard format, where pin number is determined by the
pin/port. Previously, pin numbers were based on what the Uno uses, which
does not seem to have a clear pattern.

One difference is that counting starts at port B, as there is no port A.
So PB0 is 0, PB1 is 1… PC0 is 8.

This commit also moves PWM code to the atmega328 file, as it may not be
generic to all ATmega chips.

src/examples/button/button.go

@@ -5,11 +5,9 @@ import (
 	"time"
 )
 
-// This example assumes that the button is connected to pin 8. Change the value
-// below to use a different pin.
 const (
 	led    = machine.LED
-	button = machine.Pin(8)
+	button = machine.BUTTON
 )
 
 func main() {