staging:iio:spear_adc: Add dependency on HAS_IOMEM

On archs like S390 or um this driver cannot build nor work.
Make it depend on HAS_IOMEM to bypass build failures.

drivers/staging/iio/adc/spear_adc.c: In function ‘spear_adc_probe’:
drivers/staging/iio/adc/spear_adc.c:393:2: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/Kconfig

@@ -128,6 +128,7 @@ config MXS_LRADC
 config SPEAR_ADC
 	tristate "ST SPEAr ADC"
 	depends on PLAT_SPEAR || COMPILE_TEST
+	depends on HAS_IOMEM
 	help
 	  Say yes here to build support for the integrated ADC inside the
 	  ST SPEAr SoC. Provides direct access via sysfs.