mem-cache: Create tags initialization function

mem-cache: Create tags initialization function

Having the blocks initialized in the constructor makes it harder
to apply inheritance in the tags classes. This patch decouples
the block initialization functionality from the constructor by
using an init() function. It also sets the parent cache.

Change-Id: I0da7fdaae492b1177c7cc3bda8639f79921fbbeb
Reviewed-on: https://gem5-review.googlesource.com/c/11509
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>