checkpatch: check SPDX tags in YAML files

checkpatch: check SPDX tags in YAML files

This adds a warning when a YAML file is lacking a SPDX header on first
line, or it uses incorrect commenting style.

Currently the only YAML files in the tree are Devicetree binding
documents.

Signed-off-by: Lubomir Rintel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Joe Perches <[email protected]>
Cc: Rob Herring <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>