gpiolib: remove legacy gpio_export()

There are only a handful of users of gpio_export() and
related functions.

As these are just wrappers around the modern gpiod_export()
helper, remove the wrappers and open-code the gpio_to_desc
in all callers to shrink the legacy API.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>