sim: Fixes for mremap

sim: Fixes for mremap

Remapping memory was trying to map old pages to the same new page and
calling MemState mapRegion unnecessarily. Properly increment the new
page address and remove the redundant mapRegion as remapRegion covers
its functionality.

JIRA: https://gem5.atlassian.net/browse/GEM5-475
Change-Id: Ie360755cfe488b09cbd87cd0ce525b11ac446b51
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28948
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>