To my knowledge the definition of racism is not at all dependent on how powerful/privileged the racist is compared to the race they are racist against, all that's really required to count as racism is to believe negative things about a person based purely on the race that they are. It doesn't even require any actions or harm be administered, it's purely about the beliefs the racist has toward someone. To believe that only oppressors can be racist would require a different definition of racism. The difference is that whoever is of a race that is more powerful in a given society is going to be able to more effectively turn racist beliefs into racist actions/harm as it will be harder/less acceptable to stop them. Even that scenario is simplifying things too much though.