mem-cache: Updated version of the Signature Path Prefetcher

mem-cache: Updated version of the Signature Path Prefetcher

This implementation is based in the description available in:
  Jinchun Kim, Seth H. Pugsley, Paul V. Gratz, A. L. Narasimha Reddy,
  Chris Wilkerson, and Zeshan Chishti. 2016.
  Path confidence based lookahead prefetching.
  In The 49th Annual IEEE/ACM International Symposium on Microarchitecture
  (MICRO-49). IEEE Press, Piscataway, NJ, USA, Article 60, 12 pages.

Change-Id: I4b8b54efef48ced7044bd535de9a69bca68d47d9
Reviewed-on: https://gem5-review.googlesource.com/c/14819
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>