of: Add missing 'Return' section in kerneldoc comments

Many of the DT kerneldoc comments are lacking a 'Return' section. Let's
add the section in cases we have a description of return values. There's
still some cases where the return values are not documented.

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20210325164713.1296407-8-robh@kernel.org