machine/samd21: switch UART to use new pin configuration

This allows all possible UART pin configurations to be used and avoids
some tricky configuration.

src/machine/board_itsybitsy-m0.go

@@ -55,10 +55,10 @@ const (
 
 // UART1 on the ItsyBitsy M0.
 var (
-	UART1 = UART{Bus: sam.SERCOM1_USART,
+	UART1 = UART{
 		Buffer: NewRingBuffer(),
-		Mode:   PinSERCOM,
-		IRQVal: sam.IRQ_SERCOM1,
+		Bus:    sam.SERCOM1_USART,
+		SERCOM: 1,
 	}
 )