mem-cache: Add MoveToTail to FALRU

mem-cache: Add MoveToTail to FALRU

FALRU was missing MoveToTail functionality within its invalidate
function, and MoveToHead was doing unnecessary passes when the
moved block was the head already.

Besides, added some comments to make the code understandable.

Change-Id: I2430d82b5d53c88b102a62610ea38b46d6e03a55
Reviewed-on: https://gem5-review.googlesource.com/9541
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>