machine: make machine.I2C0 and similar objects pointers

This makes it possible to assign I2C objects (machine.I2C0,
machine.I2C1, etc.) without needing to take a pointer.

This is important especially in the future when I2C may be driven using
DMA and the machine.I2C type needs to store some state.

src/machine/board_p1am-100_baremetal.go

@@ -22,7 +22,7 @@ func init() {
 
 // I2C on the P1AM-100.
 var (
-	I2C0 = I2C{
+	I2C0 = &I2C{
 		Bus:    sam.SERCOM0_I2CM,
 		SERCOM: 0,
 	}