sam: simplify I2C peripheral declarations

This commit changes the I2C declarations so that the objects are
instantiated in each chip file (e.g. machine_atsamd21e18.go) and used to
define I2C0 (and similar) in the board file (e.g. board_qtpy.go). This
should make it easier to define new board files, and reduces the need
for separate *_baremetal.go files.

I have tested this the following way:

  - With the LIS3DH driver example on the Circuit Playground Express and
    the PyBadge.
  - With the LSM6DS3 driver example on the Arduino Nano 33 IoT.

They both still work fine.

src/machine/board_atsame54-xpro.go

@@ -254,28 +254,16 @@ var (
 // I2C on the SAM E54 Xplained Pro
 var (
 	// Extension Header EXT1
-	I2C0 = I2C{
-		Bus:    sam.SERCOM3_I2CM,
-		SERCOM: 3,
-	}
+	I2C0 = sercomI2CM3
 
 	// Extension Header EXT2
-	I2C1 = I2C{
-		Bus:    sam.SERCOM7_I2CM,
-		SERCOM: 7,
-	}
+	I2C1 = sercomI2CM7
 
 	// Extension Header EXT3
-	I2C2 = I2C{
-		Bus:    sam.SERCOM7_I2CM,
-		SERCOM: 7,
-	}
+	I2C2 = sercomI2CM7
 
 	// Data Gateway Interface
-	I2C3 = I2C{
-		Bus:    sam.SERCOM7_I2CM,
-		SERCOM: 7,
-	}
+	I2C3 = sercomI2CM7
 )
 
 // SPI on the SAM E54 Xplained Pro