dev-arm: default _on_chip_memory on RealView

dev-arm: default _on_chip_memory on RealView

The _on_chip_memory member function is utilised at RealView level, but
it does not provide a default implementation. This assumes all platforms
extending RealView have on-chip memory. This patch provides a default
implementation for safeness.

Change-Id: Iaaa2bee7a85653ee97bfa95b50047eb350a88b58
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25643
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>