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.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/addac/Kconfig

@@ -35,7 +35,9 @@ config STX104
 	tristate "Apex Embedded Systems STX104 driver"
 	depends on PC104 && X86
 	select ISA_BUS_API
+	select REGMAP_MMIO
 	select GPIOLIB
+	select GPIO_REGMAP
 	help
 	  Say yes here to build support for the Apex Embedded Systems STX104
 	  integrated analog PC/104 card.