mips: update a reference to a moved Arm Document

Arm documentation has moved to Documentation/arch/arm; update a reference
in arch/mips/bmips/setup.c to match.

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>