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: 5a9b7383 ("pinctrl.txt: move it to the driver-api book")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>