mem-cache: Use Packet functions to write data blocks

mem-cache: Use Packet functions to write data blocks

Instead of using raw memcpy, use the proper writer functions
from the Packet class in Cache.

Fixed typos in comments of these functions.

Change-Id: I156a00989c6cbaa73763349006a37a18243d6ed4
Reviewed-on: https://gem5-review.googlesource.com/9661
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>