dt-bindings: arm: cpus: Add Cortex A520, A720, and X4

Add compatible strings for the Arm Cortex-A520, Cortex-A720, and
Cortex-X4 2023 CPUs.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230706205334.306483-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>