No multi-version upgrades.
You can either upgrade your 2005 cert to 2008 (if the upgrade exams haven't expired) and then upgrade 2008 to 2012, or you can start over with 2012.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability