machine: refactor pin handling to auto-detect pin mode

With this change, it's no longer necessary to set a specific pin mode:
it will get autodetected in the Configure() call.

Tested on an ItsyBitsy M4 with the mpu6050 example in the drivers repo.

src/machine/board_itsybitsy-m4.go

@@ -69,10 +69,10 @@ const (
 
 // I2C on the ItsyBitsy M4.
 var (
-	I2C0 = I2C{Bus: sam.SERCOM2_I2CM,
-		SDA:     SDA_PIN,
-		SCL:     SCL_PIN,
-		PinMode: PinSERCOM}
+	I2C0 = I2C{
+		Bus:    sam.SERCOM2_I2CM,
+		SERCOM: 2,
+	}
 )
 
 // SPI pins