dev-arm: Define a ParentMem object for DTB autogen

dev-arm: Define a ParentMem object for DTB autogen

A memory willing to autogenerate child nodes can do that directly in
the generateDeviceTree method.  However sometimes portions of memory
(child nodes) are tagged for specific applications. Hardcoding the
child node in the parent memory class is not flexible, so we delegate
this to the application model, which is registering the generator
helper via the ParentMem interface

JIRA: https://gem5.atlassian.net/browse/GEM5-768

Change-Id: I5fa5bac0decf5399dbaa3804569998dc5e6d7bc0
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34376
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>