Add support for version 3 of the ec protocol. It basically works by
stitching some additional header in front (special command code),
and having a result and packet_length stitched on for the reply.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: u-boot@lists.denx.de
Acked-by: Simon Glass <sjg@chromium.org>
Tested on snow:
Tested-by: Simon Glass <sjg@chromium.org>