dt: update a reference for reneases pcar file renamed to yaml

This file was renamed, but its reference at pfc-pinctl.txt is
still pointing to the old file.

Fixes: 7f7d408e5a00 ("dt-bindings: gpio: rcar: Convert to json-schema")
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/72d7ec[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/pinctrl/renesas,pfc-pinctrl.txt

···
    [flags]>
 
 On other mach-shmobile platforms GPIO is handled by the gpio-rcar driver.
-Please refer to Documentation/devicetree/bindings/gpio/renesas,gpio-rcar.txt
+Please refer to Documentation/devicetree/bindings/gpio/renesas,rcar-gpio.yaml
 for documentation of the GPIO device tree bindings on those platforms.