dev-arm, configs: Using _on_chip_memory for on chip memory

dev-arm, configs: Using _on_chip_memory for on chip memory

This patch is pulling the on-chip memory outside of the on_chip_devices
list.
The external interface will be more or less the same: configuration
scripts will still use the attachOnChipIO method; a new kw argument has
been added in order to store mem_ports.

We want to provide to on-chip memory the same mechanism used when
collecting on-chip dma ports. This is needed when using Ruby, since
we need to pass a non None mem_ports to prevent the bootmem to be
wired to the bus.

Change-Id: Ifc519c3072dc5de1530772c70c80dc2094e2c54c
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22000
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>