tools: ynl: support kdocs for flags in code generation

Lorenzo reports that after switching from enum to flags netdev
family lost ability to render kdoc (and the enum contents got
generally garbled).

Combine the flags and enum handling in uAPI handling.

Reported-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>