greybus: Documentation/sysfs-bus-greybus: update the bus ABI documentation

Update the ABI documentation to match the new device model.

Note that the SVC unique_id and version attributes are not yet
implemented.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>