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

drivers/iio/adc/Kconfig

@@ -134,6 +134,7 @@ config AT91_ADC
 config AT91_SAMA5D2_ADC
 	tristate "Atmel AT91 SAMA5D2 ADC"
 	depends on ARCH_AT91 || COMPILE_TEST
+	depends on HAS_IOMEM
 	help
 	  Say yes here to build support for Atmel SAMA5D2 ADC which is
 	  available on SAMA5D2 SoC family.