gpio: fix wrong checking condition for gpio range

If index++ calculates from 0, the checking condition of "while
(index++)" fails & it doesn't check any more. It doesn't follow
the loop that used at here.

Replace it by endless loop at here. Then it keeps parsing
"gpio-ranges" property until it ends.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>