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>