[PATCH] iio: adc: at91-adc: Use devm_platform_ioremap_resource

[Aishwarya R <aishwaryarj100@xxxxxxxxx>]

Use helper function to simply the code.
The devm_platform_ioremap_resource() helper in the
kernel which helps to make code cleaner a tad by
replacing few "boilerplate" lines of code with a single line.