gitattributes: consider SVG files generated

SVG files are usually created or generated using editors, e.g. Inkscape.
Exclude them from stats/diffs to improve review process by marking them
as generated.

Ref. https://github.com/github/linguist/blob/master/docs/overrides.md

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

.gitattributes

@@ -4,6 +4,10 @@
 .gitignore export-ignore
 .mailmap export-ignore
 
+# Tell git to not diff certain files
+*.svg -diff
+
 # Tell linguist that generated test pattern files should not be included in the
 # language statistics.
-*.pat linguist-generated=true
+*.pat linguist-generated
+*.svg linguist-generated