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/machine_generic.go

@@ -5,8 +5,6 @@ package machine
 // Dummy machine package that calls out to external functions.
 
 var (
-	SPI0  = SPI{0}
-	I2C0  = &I2C{0}
 	UART0 = &UART{0}
 	USB   = &UART{100}
 )
@@ -175,4 +173,13 @@ var (
 	sercomUSART3 = UART{3}
 	sercomUSART4 = UART{4}
 	sercomUSART5 = UART{5}
+
+	sercomI2CM0 = &I2C{0}
+	sercomI2CM1 = &I2C{1}
+	sercomI2CM2 = &I2C{2}
+	sercomI2CM3 = &I2C{3}
+	sercomI2CM4 = &I2C{4}
+	sercomI2CM5 = &I2C{5}
+	sercomI2CM6 = &I2C{6}
+	sercomI2CM7 = &I2C{7}
 )