mem-cache: Create ReplacementPolicy namespace

mem-cache: Create ReplacementPolicy namespace

Encapsulate the replacement policy classes in their own namespace.

As a side effect these classes have been renamed to drop the RP
suffix in the C++ code.

Change-Id: Ibb65dfb584a1413492fcf11833cf91a859cbff4e
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35795
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>