gpio: fix kernel-doc after devres.c file rename

Fix gpio kernel-doc generation after rename of the devres.c file.
Fixes these errors & warning:

Error: Cannot open file ../drivers/gpio/devres.c
Error: Cannot open file ../drivers/gpio/devres.c
WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -export ../drivers/gpio/devres.c' failed with return code 2
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/index.rst

@@ -38,7 +38,7 @@ Device tree support
 Device-managed API
 ==================
 
-.. kernel-doc:: drivers/gpio/devres.c
+.. kernel-doc:: drivers/gpio/gpiolib-devres.c
    :export:
 
 sysfs helpers