pinctrl: madera: Fix possible NULL pointer with pdata config

If we are being configured via pdata we don't necessarily have
any gpio mappings being configured that way so pdata->gpio_config
could be NULL.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>