iio: adc: sun4i-gpadc-iio: Fix module autoload when OF devices are registered

If the driver is built as a module, it won't be autloaded if the devices
are registered via OF code because the OF device table
entries are not exported as aliases

Before the patch:
$ modinfo drivers/iio/adc/sun4i-gpadc-iio.ko | grep alias
alias:          platform:sun6i-a31-gpadc-iio
alias:          platform:sun5i-a13-gpadc-iio
alias:          platform:sun4i-a10-gpadc-iio

After the patch:
$ modinfo drivers/iio/adc/sun4i-gpadc-iio.ko | grep alias
alias:          of:N*T*Callwinner,sun8i-a33-thsC*
alias:          of:N*T*Callwinner,sun8i-a33-ths
alias:          platform:sun6i-a31-gpadc-iio
alias:          platform:sun5i-a13-gpadc-iio
alias:          platform:sun4i-a10-gpadc-iio

Signed-off-by: Eduardo Molinas <edu.molinas@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/sun4i-gpadc-iio.c

@@ -712,6 +712,7 @@ static struct platform_driver sun4i_gpadc_driver = {
 	.probe = sun4i_gpadc_probe,
 	.remove = sun4i_gpadc_remove,
 };
+MODULE_DEVICE_TABLE(of, sun4i_gpadc_of_id);
 
 module_platform_driver(sun4i_gpadc_driver);