sim: Move checkpoint parameters for ptable into seperate section

sim: Move checkpoint parameters for ptable into seperate section

In checkpoint output files, the parameters for page table including
size and entries are organized not very clearly. For example:

  [system.cpu.workload]
  ...
  ptable.size=...

  [system.cpu.workload.Entry0]
  vaddr=...
  paddr=...
  flags=...

  [system.cpu.workload.Entry1]
  ...

This commit moves these parameters into a separate section named
'ptable'. For example:

  [system.cpu.workload.ptable]
  size=...

  [system.cpu.workload.ptable.Entry0]
  vaddr=...
  paddr=...
  flags=...

  [system.cpu.workload.ptable.Entry1]
  ...

Change-Id: Iaa4129b3f4f090e8c3651bde90524abba0999c7f
Signed-off-by: Ian Jiang <ianjiang.ict@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31874
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>