Merge branch 'wr-nic-unify'

This branch is unifying the wr-nic driver in wr-swithc-sw and the
one in spec-sw.  The gateware cell is the same, so we are aiming
at a single driver, maybe SDB-driven in the future.

A few differences still remain, but they are very minor details, and
we'll fix them in further commits, without a specific branch.