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