atmega2560: support UART1-3

+ example for uart

src/examples/uart/uart.go

@@ -0,0 +1,25 @@
+// This reads from UART1 and outputs to default serial, usually UART0 or USB.
+// Example of how to work with UARTs other than the default.
+package main
+
+import (
+	"machine"
+	"time"
+)
+
+var (
+	uart = machine.UART1
+	tx   = machine.UART1_TX_PIN
+	rx   = machine.UART1_RX_PIN
+)
+
+func main() {
+	uart.Configure(machine.UARTConfig{TX: tx, RX: rx})
+	for {
+		if uart.Buffered() > 0 {
+			data, _ := uart.ReadByte()
+			print(string(data))
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+}