feat(cpu): add support for Cortex-X1

This patch adds basic CPU library code to support Cortex-X1 CPU in TF-A.
Follow-up patches will add selected errata workarounds for this CPU.

Signed-off-by: Okash Khawaja <okash@google.com>
Change-Id: I4a3d50a98bf55a555bfaefeed5c7b88a35e3bc21
2 files changed