RDMA/cxgb4: Limit MRs to < 8GB for T4/T5 devices

T4/T5 hardware can't handle MRs >= 8GB due to a hardware bug.  So limit
registrations to < 8GB for thse devices.

Based on original work by Steve Wise <swise@opengridcomputing.com>.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>