mem-cache: Create Prefetcher namespace

mem-cache: Create Prefetcher namespace

Create a namespace for the Prefetcher classes.

As a side effect the Prefetcher suffix has been removed from the
C++'s classes names, and the memory leaking destructor overrides
have been fixed.

Change-Id: I9bae492d2fd4734bcdfb68c164345898e65102b2
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24537
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>