dt-bindings: arm: pmu: Add Cortex A520, A715, A720, X3, and X4

Add compatible strings for the Arm Cortex-A520, Cortex-A715,
Cortex-A720, Cortex-X3, and Cortex-X4 CPU PMUs.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230706205505.308523-1-robh@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>