gpio: pl061: document gpio-ranges property for bindings file

Document gpio-ranges property in pl061-gpio.txt

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/pl061-gpio.txt

@@ -7,4 +7,4 @@ Required properties:
   - bit 0 specifies polarity (0 for normal, 1 for inverted)
 - gpio-controller : Marks the device node as a GPIO controller.
 - interrupts : Interrupt mapping for GPIO IRQ.
-
+- gpio-ranges : Interaction with the PINCTRL subsystem.