drivers/cpu: Add generic armv8 cpu driver

drivers/cpu: Add generic armv8 cpu driver

Add a generic driver that binds to armv8 CPU nodes. The generic driver allows
- to enumerate CPUs present in a system, even when no other driver binds it
- generates ACPI SSDT code for each CPU
- Fill the ACPI MADT table (implemented in a follow up patch)

The newly introduced code could also be reused on other CPU drivers that are
compatible with armv8.

TEST: Booted on QEMU sbsa and verify the driver binds to CPU nodes.
      Confirmed with FWTS that all ACPI processor devices are present.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>