cxgbe(4): backport Chelsio T5 support from the 9-STABLE branch.

This is a direct commit to get the base 40GbE/10GbE T5 driver to
8-STABLE.  There were too many changes between 8 and 9 (t4_tom,
iw_cxgbe, etc.) and MFC'ing everything and then ripping out stuff
selectively would have been too much trouble.

Tested with "make -DMAKE_JUST_KERNELS universe".