Is equipment in data center to be considered as assets or controls?

Answer:

Theoretically, you could consider this equipment as controls only, but to be on the safe side you should consider them as assets, because that way you will be able to assess the risks related to them in a much better way.

For instance, if you considered power generator as an asset, through the risk assessment you might realize that it is not regularly maintained, that the people operating it are not trained, etc. - you probably wouldn't realize such information if you considered the power generator only as a control.