greybus: svc: add ping command

This implements the SVC "ping" command.  It's tiny and simple, but we
need something like this in order for us to "know" if all is working
well.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>