mem: Convert DRAM controller to new-style stats

mem: Convert DRAM controller to new-style stats

Note that this changes the stat format used by the DRAM
controller. Previously, it would have a structure looking a bit like
this:

  - system
    - dram: Main DRAM controller
    - dram_0: Rank 0
    - dram_1: Rank 1

This structure can't be replicated with new-world stats since stats
are confined to the SimObject name space. This means that the new
structure looks like this:

  - system
    - dram: Main DRAM controller
      - rank0: Rank 0
      - rank1: Rank 1

Change-Id: I7435cfaf137c94b0c18de619d816362dd0da8125
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21142
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Wendy Elsasser <wendy.elsasser@arm.com>