staging: iio: Fix dependencies for !HAS_IOMEM archs

Not every arch has io memory.
So, unbreak the build by fixing the dependencies.

Signed-off-by: Richard Weinberger <[e-mail]>
Acked-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/staging/iio/adc/Kconfig

···
 config AD7606
 	tristate "Analog Devices AD7606 ADC driver"
 	depends on GPIOLIB || COMPILE_TEST
+	depends on HAS_IOMEM
 	select IIO_BUFFER
 	select IIO_TRIGGERED_BUFFER
 	help