greybus: legacy: add legacy-protocol bundle driver

Add the first Greybus bundle driver that will be used when transitioning
from legacy Greybus protocols to bundle drivers.

The legacy-protocol driver initially binds to all current bundle classes.

In order to avoid having to update current module-loading scripts, keep
this driver internal to greybus core at least until modalias support is
added. Note that this prevents unloading any protocol drivers without
first tearing down the host device due to a circular module dependency.

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>