pinctrl: generic: update references to Documentation/pinctrl.txt

Update deprecated references to Documentation/pinctrl.txt since it has been
moved to Documentation/driver-api/pinctl.rst.

Signed-off-by: Ludovic Desroches <ludovic.desroches@o2linux.fr>
Fixes: 5a9b73832e ("pinctrl.txt: move it to the driver-api book")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/gpio/gpio-legacy.txt

@@ -459,7 +459,7 @@ pin controller?
 
 This is done by registering "ranges" of pins, which are essentially
 cross-reference tables. These are described in
-Documentation/pinctrl.txt
+Documentation/driver-api/pinctl.rst
 
 While the pin allocation is totally managed by the pinctrl subsystem,
 gpio (under gpiolib) is still maintained by gpio drivers. It may happen