configs: Do not assume bootmem is a System child

configs: Do not assume bootmem is a System child

As far as I can see bootmem is used in Arm systems only.

With

https://gem5-review.googlesource.com/c/public/gem5/+/21604

bootmem has been moved from system to system.realview.
This patch fix things by removing the assumption that the bootmem
SimObject lives under the system.
It is now trying to getattr a reference of the bootmem.
It is the ISA specific system builder's duty to bind the bootmem
reference (_bootmem) to the real bootmem object

Change-Id: I1e7405b5cb186de13c44bfd93fb4c8a1a5447f24
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22001
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>