iio: adc: at91-adc: Use devm_platform_ioremap_resource

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

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