docs: update some straggling Documentation/arm references

The Arm documentation has moved to Documentation/arch/arm; update the
last remaining references to match.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Samuel Holland <samuel@sholland.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> # for pwm
Signed-off-by: Jonathan Corbet <corbet@lwn.net>