checkpatch: ignore device tree related warnings

This has never been enforced and we have our own DT related checks.
Removing for now as it seems to be Linux specific.

- UNDOCUMENTED_DT_STRING
- DT_SPLIT_BINDING_PATCH
- DT_SCHEMA_BINDING_PATCH

Signed-off-by: Anas Nashif <anas.nashif@intel.com>