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>