dt-bindings: pinctrl: sirf: Fix typo abitrary

Fix comment typo.
s/abitrary/arbitrary/

Signed-off-by: Naoki Hayama <naoki.hayama@lineo.co.jp>
Link: https://lore.kernel.org/r/614fe604-ab8e-21cb-0c3a-db6ddeff2a4e@lineo.co.jp


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