[PATCH] dts: update hinted return types of various methods

[Luca Vizzarro]

Hi Andrew,

you may have noticed that the CI failed. It is worth running the 
following scripts prior to submitting:

- devtools/dts-check-format.py (in a Poetry environment)
- devtools/checkpatches.sh
- devtools/check-git-log.sh

mypy specifically fails in some occasions because of missed faults as a 
consequence of disabled type checking. It'd be really appreciated if you 
could also tackle these type issues with your patch.

Best,
Luca