gpio: gpio-max710x: Support device tree probing

For probing via device tree, we need to support the case without platform_data.
In this case, chip.base is set to -1 for automatic numbering.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>