Fill the MADT table in the GIC driver and armv8 CPU driver to
drop SoC specific code. While the GIC only needs devicetree
data, the CPU driver needs additional information stored in
the cpu_plat struct.

While on it update the only board making use of the existing
drivers and writing ACPI MADT in mainboard code.

TEST: Booted on QEMU sbsa-ref using GICV3 driver model generated MADT.
      Booted on QEMU raspb4 using GICV2 driver model generated MADT.

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

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>