mem: Teach SimpleMem to return a MemBackdoor when appropriate.

mem: Teach SimpleMem to return a MemBackdoor when appropriate.

If the back door SimpleMem inherits from AbstractMem has a pointer and
is hence valid, SimpleMem will return that pointer when asked.

Change-Id: I734daba48e4ae5b4ad8ac9a108e7b12b5e82803f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17669
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>