iio: addac: stx104: Migrate to the regmap API

The regmap API supports IO port accessors so we can take advantage of
regmap abstractions rather than handling access to the device registers
directly in the driver.

In addition, to improve code organization in stx104_probe(), the
devm_iio_device_register() call is moved above GPIO configuration in
order to keep relevant code closer together.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/0bcdfc4738cc019fb2ff83f61eb46a3488bc166d.1680790580.git.william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>