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>

.checkpatch.conf

@@ -21,4 +21,7 @@
 --ignore SPDX_LICENSE_TAG
 --ignore C99_COMMENT_TOLERANCE
 --ignore REPEATED_WORD
+--ignore UNDOCUMENTED_DT_STRING
+--ignore DT_SPLIT_BINDING_PATCH
+--ignore DT_SCHEMA_BINDING_PATCH
 --exclude ext