MFC r256694, r256713, r256714.

r256694:
iw_cxgbe: iWARP driver for Chelsio T4/T5 chips.  This is a straight port
of the iw_cxgb4 found in OFED distributions.

r256713:
iw_cxgbe should have a dependency on t4nex.

r256714:
Fix typo in previous commit.

Approved by:	re (hrs)
MFC r256467:

Delete all of the old RDMA code (except krping, which was switched to
use sys/ofed some time back).  This has been sitting around as dead code
in the tree for a very long time.

Approved by:	re (hrs)