nvme : Add sysfs entry for NVMe CMBs when appropriate

Add a sysfs attribute that contains salient information about the NVMe
Controller Memory Buffer when one is present. For now, just display the
information about the CMB available from the control registers. We attach
the CMB attribute file to the existing nvme_ctrl sysfs group so it can
handle the sysfs teardown.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Jay Freyensee <james_p_freyensee@linux.intel.com>
Signed-off-by: Stephen Bates <sbates@raithlin.com>
Acked-by Jon Derrick: <jonathan.derrick@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>