stdlib: Give board interface for mem ports

It is possible that the board has more than just a "main" memory. For
instance, the ArmBoard has a boot memory which is separate from the
`get_memory` function.

This moves the `get_mem_ports` function to the board so that the board
can optionally override it.

Change-Id: I05e388cc93e691e9a4fa674023f158af447349f9
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/64631
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed