mem: Make DRAMCtrl::decodeAddr const

mem: Make DRAMCtrl::decodeAddr const

DRAMCtrl's decodeAddr does not need to modify the packet it
receives, nor should it modify the contents of the class,
and therefore both the packet and the function are made const.

Change-Id: I577f48d9a43611ba54878a9a793cb7b4fbb326f4
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17540
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>