Add cad(4), a driver for Cadence GEM.

Add cad(4), a driver for Cadence GEM.

This initial revision targets the Zynq-7000, where the GEM implements
single transmit and receive queues with 32-bit DMA addresses. The driver
uses receive checksum offload, but transmit checksum offload is disabled
because of a hardware quirk. Also, the hardware's receive path is prone
to getting stuck if input cannot be handled quickly enough. The driver
attempts to recover by restarting the receiver when no input has been
seen for a while.

OK kettenis@