iio: adc: sun4i-gpadc-iio: Use devm_platform_ioremap_resource

Use the helper function that wraps the calls to platform_get_resource()
and devm_ioremap_resource() together.  Reduces boilerplate and suggested
by coccinelle.

Signed-off-by: Aishwarya Ramakrishnan <aishwaryarj100@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>