greybus: Explicitly add pad-bytes to manifest descriptors

Explicitly add pad-bytes to manifest descriptors to match their layout
in greybus specification.

Reviewed-by: Alex Elder <elder@linaro.org>
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>