greybus: introduce protocol abstraction

greybus: introduce protocol abstraction

Define a protocol structure that will allow protocols to be
registered dynamically.  For now we just introduce a bookkeeping
data structure.  Upcoming patches will move protocol-related methods
into the protocol structure, and will start registering protocol
handlers dynamically.

A list of connections using a given protocol is maintained so we can
tell when a protocol is no longer in use.  This may not be necessary
(we could use a kref instead) but it may turn out to be a good way
to clean things up.

The interface is gb_protocol_get() and gb_protocol_put() for a
connection, allowing the protocol to be looked up and the connection
structure to be inserted into its list.

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