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_pyportal_baremetal.go

@@ -4,22 +4,8 @@ package machine
 
 import (
 	"device/sam"
-	"runtime/interrupt"
 )
 
-var (
-	UART1  = &_UART1
-	_UART1 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    sam.SERCOM4_USART_INT,
-		SERCOM: 4,
-	}
-)
-
-func init() {
-	UART1.Interrupt = interrupt.New(sam.IRQ_SERCOM4_2, _UART1.handleInterrupt)
-}
-
 // I2C on the PyPortal.
 var (
 	I2C0 = &I2C{
@@ -36,7 +22,3 @@ var (
 	}
 	NINA_SPI = SPI0
 )
-
-var (
-	DefaultUART = UART1
-)