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/machine/board_arduino.go

@@ -7,21 +7,39 @@ func CPUFrequency() uint32 {
 	return 16000000
 }
 
+// Digital pins, marked as plain numbers on the board.
+const (
+	D0  = PD0 // RX
+	D1  = PD1 // TX
+	D2  = PD2
+	D3  = PD3
+	D4  = PD4
+	D5  = PD5
+	D6  = PD6
+	D7  = PD7
+	D8  = PB0
+	D9  = PB1
+	D10 = PB2
+	D11 = PB3
+	D12 = PB4
+	D13 = PB5
+)
+
 // LED on the Arduino
-const LED Pin = 13
+const LED Pin = D13
 
 // ADC on the Arduino
 const (
-	ADC0 Pin = 0
-	ADC1 Pin = 1
-	ADC2 Pin = 2
-	ADC3 Pin = 3
-	ADC4 Pin = 4 // Used by TWI for SDA
-	ADC5 Pin = 5 // Used by TWI for SCL
+	ADC0 Pin = PC0
+	ADC1 Pin = PC1
+	ADC2 Pin = PC2
+	ADC3 Pin = PC3
+	ADC4 Pin = PC4 // Used by TWI for SDA
+	ADC5 Pin = PC5 // Used by TWI for SCL
 )
 
 // UART pins
 const (
-	UART_TX_PIN Pin = 1
-	UART_RX_PIN Pin = 0
+	UART_TX_PIN Pin = PD1
+	UART_RX_PIN Pin = PD0
 )