mbrola/de*: support the /r\/ consonant

The [R] consonant is used to realize the /r\/ sound in de1-de5 [1].
The de6 and de7 voices have functional /r\/ phonemes, so don't
need this mapping.

[1] The de4 voice has support for [r\], but this support is not
    complete, so words like /str\Vt/ don't work. As such, this
    voice uses the [R] realization of /r\/.

    Ideally, the [R] phoneme should only be used by the voice when
    [r\] does not work (e.g. use [R6] for /r\V/ instead of [r\6]).