MAINTAINERS: belong Documentation/pinctrl.txt properly

I'm pretty sure that Documentation/pinctrl.txt would be better maintained by
pinctrl subsystem.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

MAINTAINERS

@@ -8959,6 +8959,7 @@ L:	linux-gpio@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl.git
 S:	Maintained
 F:	Documentation/devicetree/bindings/pinctrl/
+F:	Documentation/pinctrl.txt
 F:	drivers/pinctrl/
 F:	include/linux/pinctrl/