patman: Check patches in parallel

For large series this can take a while. Run checkpatch in parallel to
try to reduce the time. The checkpatch information is still reported in
sequential order, so a very slow patch at the start can still slow
things down. But overall this gives good results.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>