mem-cache: Implement FPC cache compressor

mem-cache: Implement FPC cache compressor

Implementation of Frequent Pattern Compression, proposed
by Alameldeen et al. in "Frequent Pattern Compression: A
Significance-Based Compression Scheme for L2 Caches".

Change-Id: I6dca8ca6b3043b561140bc681dbdbe9f7cef27d7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36395
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>