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_generic.go

@@ -165,3 +165,14 @@ func uartRead(bus uint8, buf *byte, bufLen int) int
 
 //export __tinygo_uart_write
 func uartWrite(bus uint8, buf *byte, bufLen int) int
+
+// Some objects used by Atmel SAM D chips (samd21, samd51).
+// Defined here (without build tag) for convenience.
+var (
+	sercomUSART0 = UART{0}
+	sercomUSART1 = UART{1}
+	sercomUSART2 = UART{2}
+	sercomUSART3 = UART{3}
+	sercomUSART4 = UART{4}
+	sercomUSART5 = UART{5}
+)