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_feather-m0.go

@@ -4,7 +4,6 @@ package machine
 
 import (
 	"device/sam"
-	"runtime/interrupt"
 )
 
 // used to reset into bootloader
@@ -55,18 +54,7 @@ const (
 )
 
 // UART1 on the Feather M0.
-var (
-	UART1  = &_UART1
-	_UART1 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    sam.SERCOM1_USART,
-		SERCOM: 1,
-	}
-)
-
-func init() {
-	UART1.Interrupt = interrupt.New(sam.IRQ_SERCOM1, _UART1.handleInterrupt)
-}
+var UART1 = &sercomUSART1
 
 // I2C pins
 const (