sam: simplify definition of SERCOM UART peripherals

Instead of defining them separately for each board, define them once in
the chip definition and later simply use &sercomUART1 etc. to refer to
them. This is simpler and less error-prone.

I found two bugs while working on this:

  - The P1AM-100 board mixed SERCOM 5 and SERCOM 3. It looks like SERCOM
    5 was intended, based on the used pins.
  - The Adafruit Matrix Portal appears to have configured the wrong
    interrupt.

Unfortunately, I can't test these fixes. However, they make it clear
that such a change is important to avoid bugs.

I tested this commit on the PyBadge and the Circuit Playground Express.

src/machine/machine_atsamd21.go

@@ -504,10 +504,6 @@ type UART struct {
 	Interrupt interrupt.Interrupt
 }
 
-var (
-	USB = &USBCDC{Buffer: NewRingBuffer()}
-)
-
 const (
 	sampleRate16X = 16
 	lsbFirst      = 1
@@ -1743,6 +1739,10 @@ type USBCDC struct {
 	sent              bool
 }
 
+var (
+	USB = &USBCDC{Buffer: NewRingBuffer()}
+)
+
 const (
 	usbcdcTxSizeMask          uint8 = 0x3F
 	usbcdcTxBankMask          uint8 = ^usbcdcTxSizeMask