greybus: introduce SVC protocol

This patch adds support for the Greybus SVC protocol.  We may want
to rearrange protocol numbers at some point, since this is a pretty
fundamental protocol.

Note:  It has only been compile tested; no SVC CPorts have yet been
defined, so this code is not yet exercised.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>