io/i2c: document that devfs requires the i2c-dev kernel module
io/i2c: document that devfs requires the i2c-dev kernel module

Change-Id: Id532f4d5ca331c6ebe0f59b891b93c4876f126b6
Reviewed-on: https://go-review.googlesource.com/21933
Reviewed-by: Jaana Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

golang/exp,golang/exp,golang/exp