mem-cache: Insert on block allocation

mem-cache: Insert on block allocation

When a block is being replaced in an allocation, if successfull,
the block will be inserted. Therefore we move the insertion
functionality to allocateBlock().

allocateBlock's signature has been modified to allow this
modification.

Change-Id: I60d17a83ff4f3021fdc976378868ccde6c7507bc
Reviewed-on: https://gem5-review.googlesource.com/10812
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>