Four things here:
1.) Don't make it a method on *mysqlDriver. To you this, you must import the mysql-package non-anonymously anyways. It makes no sense to indirect access to this over the sql package. Just make it a exported function.
2.) Please name it consistently with the infile functions, so `RegisterTLSConfig` here.
3.) A deregister function is missing.
4.) Please add a short documentation to the function. A short example how to use this (like https://github.com/go-sql-driver/mysql/blob/master/utils.go#L30 ) would be extra nice.