iio: adc: Fix build error of missing devm_ioremap_resource on UM

The devres.o gets linked if HAS_IOMEM is present so on ARCH=um
allyesconfig (COMPILE_TEST) failed with:

drivers/built-in.o: In function `at91_adc_probe':
at91-sama5d2_adc.c:(.text+0x48f548): undefined reference to `devm_ioremap_resource'

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>