pinctrl: introduce pinctrl_gpio_get_config()

pinctrl: introduce pinctrl_gpio_get_config()

This is a counterpart of pinctrl_gpio_set_config(), which will be used
to implement the ->get() interface in a GPIO driver for SCMI.

This also requires that we create a stub so pin_config_get_for_pin()
can build when CONFIG_PINCONF is disabled.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>