.checkpatch.conf: Ignore a few more warnings

- PRINTK_WITHOUT_KERN_LEVEL is a new warning that coreboot's printks
don't follow.
- UNDOCUMENTED_DT_STRING is ignored because we don't have any
documentation for .dt files.
- SPDX_LICENSE_TAG is another new warning.  We can evaluate adding the
spdx license tags to our files, but at this point they aren't there, so
disable it for now.

Change-Id: I4b7fcfc0339b358a48e77188d85b47c022c7eb49
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/28044
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

.checkpatch.conf

@@ -17,6 +17,9 @@
 --ignore CONFIG_DESCRIPTION
 --ignore MISSING_SPACE
 --ignore CORRUPTED_PATCH
+--ignore SPDX_LICENSE_TAG
+--ignore UNDOCUMENTED_DT_STRING
+--ignore PRINTK_WITHOUT_KERN_LEVEL
 
 # FILE_PATH_CHANGES seems to not be working correctly. It will
 # choke on added / deleted files even if the MAINTAINERS file