Silicon/AMD/StyxDtbLoaderLib: use Cortex-A57 IDs instead of generic ARMv8

Use the more precise Cortex-A57 based compatible strings to describe
the CPUs and the PMUs in the device tree. For compatibility with
existing users, leave the current values as fallback aliases.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>