mfd: Add ChromeOS EC messages header

This file is included verbatim from the ChromeOS EC respository.
Ideally we would prefer to avoid changing it, to make it easier
to track this rapidly-changing file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>