mfd: madera: Fix bad reference to pinctrl.txt file

The pinctrl.txt file was converted into reStructuredText and moved into
driver-api folder. This patch updates the broken reference.

Fixes: 5a9b7383 ("pinctrl.txt: move it to the driver-api book")
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

include/linux/mfd/madera/pdata.h

@@ -34,7 +34,8 @@ struct madera_codec_pdata;
  * @micvdd:	    Substruct of pdata for the MICVDD regulator
  * @irq_flags:	    Mode for primary IRQ (defaults to active low)
  * @gpio_base:	    Base GPIO number
- * @gpio_configs:   Array of GPIO configurations (See Documentation/pinctrl.txt)
+ * @gpio_configs:   Array of GPIO configurations (See
+ *		    Documentation/driver-api/pinctl.rst)
  * @n_gpio_configs: Number of entries in gpio_configs
  * @gpsw:	    General purpose switch mode setting. Depends on the external
  *		    hardware connected to the switch. (See the SW1_MODE field