Add back '*_t' dynamic field for single valued text fields

Solr used to have a '_t' dynamic field which was single valued and a "_txt" field for multi-valued text 

Solr 4.x : https://github.com/apache/lucene-solr/blob/branch_4x/solr/example/example-schemaless/solr/collection1/conf/schema.xml#L129

Somewhere in Solr 5.x both became the same definition . https://github.com/apache/lucene-solr/blob/branch_5_4/solr/server/solr/configsets/data_driven_schema_configs/conf/managed-schema#L138

In master now there is no "_t" dynamic field anymore. 

We have a single-valued dynamic field and multi-valued dynamic field for ints, longs, boolean, float, date , string . We should provide the same option for a text field