io/spi add a no-implementation of devfs for !linux

Allow users to build with Devfs on all platforms by exporting required
symbols with no implementation.
Users can use tools such as goimports even if their host machine is not linux.

Change-Id: Iad30774e4344774a244dc2f30a767f6243782edd
Reviewed-on: https://go-review.googlesource.com/23923
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
2 files changed