arch-arm: default MIDR for Armv8 ISA processors

arch-arm: default MIDR for Armv8 ISA processors

Software such as Trusted Firmware-A checks the MIDR register
to identify which core model is present in the platform.
The previous default value referred to a Cortex-A15 Armv7-A
processor, however when AArch64 is enabled, an Armv8 processor
is expected.
This patch assigns the Cortex-A57 MIDR if AArch64 is enabled.

Change-Id: Id1677a77d2f04843423f7b013405445f3d253399
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22846
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>