iio:adc:stm32-adc: Add HAS_IOMEM dependency

Seems that there are config combinations in which this driver gets enabled
and hence selects the MFD, but with out HAS_IOMEM getting pulled in
via some other route.  MFD is entirely contained in an
if HAS_IOMEM block, leading to the build issue in this bugzilla.

https://bugzilla.kernel.org/show_bug.cgi?id=209889

Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210124195034.22576-1-jic23@kernel.org

drivers/iio/adc/Kconfig

@@ -925,6 +925,7 @@ config STM32_ADC_CORE
 	depends on ARCH_STM32 || COMPILE_TEST
 	depends on OF
 	depends on REGULATOR
+	depends on HAS_IOMEM
 	select IIO_BUFFER
 	select MFD_STM32_TIMERS
 	select IIO_STM32_TIMER_TRIGGER