cxgb4: add new ULD type CXGB4_ULD_ISCSIT

Chelsio iSCSI target offload driver
will register with cxgb4 driver as ULD of type
CXGB4_ULD_ISCSIT.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Acked-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>