Fix comment typo.
s/abitrary/arbitrary/

Signed-off-by: Naoki Hayama <naoki.hayama@lineo.co.jp>
Link: https://lore.kernel.org/r/7d1856e2-84c7-ab19-863d-2d500569d58c@lineo.co.jp


Signed-off-by: Rob Herring <robh@kernel.org>