dt-bindings: arm: Drop obsolete cavium-thunder2.txt

The binding is already converted to schema and is located in
Documentation/devicetree/bindings/arm/bcm/brcm,vulcan-soc.yaml.

Link: https://lore.kernel.org/r/20250806212812.1634740-1-robh@kernel.org


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