greybus: connection: serialise connection creation

Serialise connection creation against concurrent creation and
destruction using a global mutex.

This is needed to prevent two drivers from attempting to create a
connection to the same interface CPort and to cope with a racing
connection destroy when moving to driver managed connections.

Note that the locking can not (easily) be made more fine-grained as not
all connections have an interface, but these are not hot paths anyway.

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