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-m4-can.go

@@ -4,7 +4,6 @@ package machine
 
 import (
 	"device/sam"
-	"runtime/interrupt"
 )
 
 // used to reset into bootloader
@@ -104,25 +103,12 @@ var (
 )
 
 var (
-	UART1  = &_UART1
-	_UART1 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    sam.SERCOM5_USART_INT,
-		SERCOM: 5,
-	}
+	UART1 = &sercomUSART5
 
-	UART2  = &_UART2
-	_UART2 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    sam.SERCOM0_USART_INT,
-		SERCOM: 0,
-	}
+	UART2 = &sercomUSART0
 )
 
 func init() {
-	UART1.Interrupt = interrupt.New(sam.IRQ_SERCOM5_2, _UART1.handleInterrupt)
-	UART2.Interrupt = interrupt.New(sam.IRQ_SERCOM0_2, _UART2.handleInterrupt)
-
 	// turn on neopixel
 	D7.Configure(PinConfig{Mode: PinOutput})
 	D7.High()