mem,sim: Get the page size from the page table in SE mode.

mem,sim: Get the page size from the page table in SE mode.

The page table already knows the size of a page without having to
directly use any ISA specific constants.

Change-Id: I68b575e194697065620a2097d972076886766f74
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34172
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>