The backup memory port doesn't work on multi-channel configurations, it
just screws up the Nasti tag bits. This patch always instantiates a
single-channel backup memory port, which relies on the memory channel
selector to only enable a single memory channel when the backup memory
port is enabled. There are some assertions to make sure this happens,
as otherwise memory gets silently corrupted.
While this is a bit of a hack, the backup memory port will be going away
soon so I don't want to spend a whole lot of time fixing it. The
generated hardware is actually very similar: we used to elaborate a
Nasti arbiter inside the backup memory support, now there's one outside
of it instead.
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