rtc: enable RTC framework on ARCH=um

There's no real reason it should be disabled, and at least we can
use it for development & testing with the RTC test driver.

However, two devices are missing a HAS_IOMEM dependency, so add
that to avoid build failures from e.g. allyesconfig.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201120211103.6895ac740d11.Ic19a9926e8e4c70c03329e55f9e5b1d45095b904@changeid