Doctrine MySqlPlatform can't enum type

This type is specific to MySql and not supported by doctrine and owncloud.
Other software using the same DB can have tables with this type, so we need
to handle this when reading the tables.

See also http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/cookbook/mysql-enums.html
Fixes #6915
@DeepDiver1975 @PVince81