SELECT Id, Name FROM SBQQ__Quote__c WHERE SBQQ__Opportunity2__r.Name = 'OppName'

With bind variable it will be: SELECT Id, Name FROM SBQQ__Quote__c WHERE SBQQ__Opportunity2__r.Name=:varName

Reference: Salesforce CPQ interaction with Opportunity Fields

"A Quote will be related to the Opportunity if the Quote's Opportunity Lookup field (SBQQ__Opportunity2__c) is populated with the Opportunity."

Also: CPQ Object Relationships (salesforce.com)