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/board_pybadge.go

@@ -84,6 +84,13 @@ const (
 	UART2_RX_PIN = A5
 )
 
+var (
+	UART1 = &sercomUSART5
+	UART2 = &sercomUSART0
+
+	DefaultUART = UART1
+)
+
 // I2C pins
 const (
 	SDA_PIN = PA12 // SDA: SERCOM2/PAD[0]