[SCSI] SAS transport class

The SAS transport class contains common code to deal with SAS HBAs, an
aproximated representation of SAS topologies in the driver model,
and various sysfs attributes to expose these topologies and managment
interfaces to userspace.

In addition to the basic SCSI core objects this transport class introduces
two additional intermediate objects:  The SAS PHY as represented by struct
sas_phy defines an "outgoing" PHY on a SAS HBA or Expander, and the SAS
remote PHY represented by struct sas_rphy defines an "incoming" PHY on a
SAS Expander or end device.  Note that this is purely a software concept, the
underlying hardware for a PHY and a remote PHY is the exactly the same.

There is no concept of a SAS port in this code, users can see what PHYs
form a wide port based on the port_identifier attribute, which is the same
for all PHYs in a port.

This submission doesn't handle hot-plug addition or removal of SAS devices
and thus doesn't do scanning in a workqueue yet, that will be added...