[BUGFIX] No RTE Transformation for subtype_value_field-fields of plugins

The RTE transformation depends on the result of BackendUtility
getTCAtypes. If the RTE field is in the subtypes_addlist of a plugin
the transformation is not performed. The patch adds the related
subtype fields to the return value.

Resolves: #51901
Releases: 6.2, master
Change-Id: I8bba3d03b5187b06fec5a9b5a0e151466d814d60
Reviewed-on: http://review.typo3.org/35139
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>