gpio: intel-mid: Fix build warning when !CONFIG_PM

The only usage of function intel_gpio_runtime_idle() is here (in the
same file):

static const struct dev_pm_ops intel_gpio_pm_ops = {
	SET_RUNTIME_PM_OPS(NULL, NULL, intel_gpio_runtime_idle)
};

And when CONFIG_PM is not set, the macro SET_RUNTIME_PM_OPS expands to
nothing, causing the following compiler warning:

drivers/gpio/gpio-intel-mid.c:324:12: warning: ‘intel_gpio_runtime_idle’
defined but not used [-Wunused-function]
static int intel_gpio_runtime_idle(struct device *dev)

Fix it by annotating the function with __maybe_unused.

Signed-off-by: Augusto Mecking Caringi <augustocaringi@gmail.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>