machine: make UART objects pointer receivers

This means that machine.UART0, machine.UART1, etc are of type
*machine.UART, not machine.UART. This makes them easier to pass around
and avoids surprises when they are passed around by value while they
should be passed around by reference.

There is a small code size impact in some cases, but it is relatively
minor.

src/machine/board_atsame54-xpro.go

@@ -240,28 +240,32 @@ var (
 // UART on the SAM E54 Xplained Pro
 var (
 	// Extension Header EXT1
-	UART1 = UART{
+	UART1  = &_UART1
+	_UART1 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    sam.SERCOM0_USART_INT,
 		SERCOM: 0,
 	}
 
 	// Extension Header EXT2
-	UART2 = UART{
+	UART2  = &_UART2
+	_UART2 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    sam.SERCOM5_USART_INT,
 		SERCOM: 5,
 	}
 
 	// Extension Header EXT3
-	UART3 = UART{
+	UART3  = &_UART3
+	_UART3 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    sam.SERCOM1_USART_INT,
 		SERCOM: 1,
 	}
 
 	// EDBG Virtual COM Port
-	UART4 = UART{
+	UART4  = &_UART4
+	_UART4 = UART{
 		Buffer: NewRingBuffer(),
 		Bus:    sam.SERCOM2_USART_INT,
 		SERCOM: 2,
@@ -269,10 +273,10 @@ var (
 )
 
 func init() {
-	UART1.Interrupt = interrupt.New(sam.IRQ_SERCOM0_2, UART1.handleInterrupt)
-	UART2.Interrupt = interrupt.New(sam.IRQ_SERCOM5_2, UART2.handleInterrupt)
-	UART3.Interrupt = interrupt.New(sam.IRQ_SERCOM1_2, UART3.handleInterrupt)
-	UART4.Interrupt = interrupt.New(sam.IRQ_SERCOM2_2, UART4.handleInterrupt)
+	UART1.Interrupt = interrupt.New(sam.IRQ_SERCOM0_2, _UART1.handleInterrupt)
+	UART2.Interrupt = interrupt.New(sam.IRQ_SERCOM5_2, _UART2.handleInterrupt)
+	UART3.Interrupt = interrupt.New(sam.IRQ_SERCOM1_2, _UART3.handleInterrupt)
+	UART4.Interrupt = interrupt.New(sam.IRQ_SERCOM2_2, _UART4.handleInterrupt)
 }
 
 // I2C on the SAM E54 Xplained Pro