cxgbe: Move the STAG and PBL memory pool arenas to the base driver

Both RDMA (iw_cxgbe) and NVMe offloads use TPT table entries to map
transaction tags in incoming PDUs to buffers in host memory permitting
direct placement of received data into host memory buffers avoiding
copies (iSCSI offload uses a different scheme for mapping tags to host
memory). Move the vmem arenas for the supporting card memory regions
from iw_cxgbe to the main driver so they can be shared with the NVMe
offload driver. In addition, add some helper routines for
constructing work requests to update TPT table entries.

MFC after: 3 days
Sponsored by: Chelsio Communications