documentation: pinctrl bindings: Fix trivial typo 'abitrary'

A misspelled 'arbitrary' propagated to quite a few locations in the DT
binding documentation for pin-controllers. Fixing by:
  git grep abitrary | cut -f1 -d: | xargs sed -i 's/abitrary/arbitrary/'

Reported-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Rob Herring <robh@kernel.org>