arch-arm, configs: Treat the bootloader rom as cacheable memory

arch-arm, configs: Treat the bootloader rom as cacheable memory

Prior to this changeset the bootloader rom (instantiated as a
SimpleMemory) in ruby Arm systems was treated as an IO device and it
was fronted by a DMA controller. This changeset moves the bootloader
rom and adds it to the system as another memory with a dedicated
directory controller.

Change-Id: I094fed031cdef7f77a939d94f948d967b349b7e0
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8741
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>