ATSC A/53 CC support missing from nvenc_h264 encoder

[unsword]

Other H.264 encoders have ATSC A/53 CC copy from source support which was added with this commit:
​https://github.com/FFmpeg/FFmpeg/commit/e06114fed3afa69187b3dfc09a7a1a25cfd558b3

However the required option -a53cc is not present for the nvenc_h264 encoder

Looks like there was a patch submitted to add A/53 to the nvenc encoder but not implemented ?
https://ffmpeg.org/pipermail/ffmpeg-devel/2016-June/194927.html

Summary of the bug:
How to reproduce:

ffmpeg -h encoder=nvenc_h264

Patches should be submitted to the ffmpeg-devel mailing list and not this bug tracker.