dcache: Snoop writes to memory by other agents

dcache: Snoop writes to memory by other agents

This adds a path where the wishbone that goes out to memory and I/O
also gets fed back to the dcache, which looks for writes that it
didn't initiate, and invalidates any cache line that gets written to.

This involves a second read port on the cache tag RAM for looking up
the snooped writes, and effectively a second write port on the cache
valid bit array to clear bits corresponding to snoop hits.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>