dev-arm: Fix VExpressFastmodel timer configs

generic_timer is no longer in the return value of _on_chip_devices. We
should correct the _on_chip_devices. Furthermore, to prevent the timer
conflict with the fastmodel, we should remove unwanted timer.

Change-Id: I6ec7f9749546df3e8f125a5b96e7ed83cab2ea56
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36379
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
2 files changed