The kernel-doc for the DT APIs are not included in the documentation
build. Add them.

Cc: Frank Rowand
Cc: Mauro Carvalho Chehab
Signed-off-by: Rob Herring
Reviewed-by: Mauro Carvalho Chehab
Link: https://lore.kernel.org/r/20210325164713.1296407-9-robh@kernel.org