cros_ec: Move EC interface into common library

Add a common library for obtaining access to the Chrome OS EC. This is
used by boards which need to talk to the EC.

Reviewed-by: Vadim Bendebury <vbendeb@google.com>
Tested-by: Vadim Bendebury <vbendeb@google.com>
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>