dt-bindings: Fix generated example files getting added to schemas

Commit 837158b8 ("dt-bindings: Check the examples against the
schemas") started generating YAML encoded DT files to validate the
examples against the schema. When running 'make dt_binding_check' in
tree after the 1st time, the generated example .dt.yaml files are
mistakenly added to the list of schema files. Exclude *.example.dt.yaml
files from the search for schema files.

Fixes: 837158b8 ("dt-bindings: Check the examples against the schemas")
Reported-by: Guido Günther <agx@sigxcpu.org>
Tested-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Rob Herring <robh@kernel.org>