mem-cache: Add lookup latency to access' whenReady

mem-cache: Add lookup latency to access' whenReady

When dealing with writebacks, as soon as the packet metadata arrives
there will be a tag lookup, done sequentially because a write can't
be done in parallel. While the tag lookup is being done, the payload
will arrive. When both the payload are present and the tag is correct
block entry is determined the fill happens.

Change-Id: If1a0085d742458b675bfc012b6d908d9d9a25e32
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/14877
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>