config: Add flag options to set the hardware prefetchers to use

This patch adds three flag options to set the prefetcher class of the
L1i cache, L1d cache and L2 cache.

Change-Id: I310fcd9c49f9554d98cd565a32bdb96a3e165486
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17709
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>