dt-bindings: arm,coresight-cti: Drop type for 'cpu' property

'cpu' has been added as a single phandle type to dtschema, so drop the
type here.

Reviewed-by: Mike Leach <mike.leach@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230925220511.2026514-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>