On Hurricane we want to be able to support multiple memory channels but have a
fallback to fewer, since the full configuration is going to require a
complicated FPGA setup. This adds another sort of interconnect that can switch
between having different numbers of top-level memory channels active at chip
boot time.
This interconnect is a bit funny: changing the select input when there is
memory traffic is a bad idea. This is fine for this use case, since we really
only care about changing the memory configuration at boot time -- since it'll
scramble the memory of the machine it's not so useful, anyway.
The advantage is that we don't have to have a full 8x8 Nasti crossbar in our
chip, which would be fairly expensive. Changing the crossbar would garble
memory as well, so it's not like it would add any extra functionality.
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