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_circuitplay_express.go

@@ -83,6 +83,12 @@ const (
 	SCL1_PIN = PA01 // I2C1 internal
 )
 
+// I2C on the Circuit Playground Express.
+var (
+	I2C0 = sercomI2CM5 // external device
+	I2C1 = sercomI2CM1 // internal device
+)
+
 // SPI pins (internal flash)
 const (
 	SPI0_SCK_PIN = PA21 // SCK: SERCOM3/PAD[3]