greybus: operation: Move operation header to greybus_protocols.h

This should be exposed to external users (like gbsim). Move it to
greybus_protocols.h.

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