pinctrl: lynxpoint: Keep pointer to struct device instead of its container

There is no need to keep pointer to struct platform_device, which is container
of struct device, because the latter is what have been used everywhere outside
of ->probe() path. In any case we may derive pointer to the container when
needed.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>