mem-cache: Isolate prefetching bit

mem-cache: Isolate prefetching bit

Previously the prefetching bit was among the status bits;
yet, it has no correlation with the other bits. It has
been isolated as a single boolean, with a respective getter
and setter.

Change-Id: Ibe76e1196ca17a7c9ab9bda2216186707427cb64
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35699
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>