greybus: svc: Implement DME peer get/set attributes helpers

These are required to get/set DME attributes of the modules. This is
implemented based on the greybus specifications.

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