mem-cache: Undefine compression ratio of perfect compression

mem-cache: Undefine compression ratio of perfect compression

Commit c0d67b2263aab6a729368373d9cdef9883870241 assumes that the
cache contains a parameter for its compression ratio. This is not
the case upstream, so force the user to provide it instead.

Change-Id: Ic7b4878bede6b0a34e4adfe7e0aa65a0ee48d1f6
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36495
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>