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

drivers/rtc/Kconfig

@@ -13,7 +13,7 @@ config RTC_MC146818_LIB
 menuconfig RTC_CLASS
 	bool "Real Time Clock"
 	default n
-	depends on !S390 && !UML
+	depends on !S390
 	select RTC_LIB
 	help
 	  Generic RTC class support. If you say yes here, you will
@@ -1019,6 +1019,7 @@ config RTC_DRV_DS1553
 
 config RTC_DRV_DS1685_FAMILY
 	tristate "Dallas/Maxim DS1685 Family"
+	depends on HAS_IOMEM
 	help
 	  If you say yes here you get support for the Dallas/Maxim DS1685
 	  family of real time chips.  This family includes the DS1685/DS1687,
@@ -1152,6 +1153,7 @@ config RTC_DRV_STK17TA8
 
 config RTC_DRV_M48T86
 	tristate "ST M48T86/Dallas DS12887"
+	depends on HAS_IOMEM
 	help
 	  If you say Y here you will get support for the
 	  ST M48T86 and Dallas DS12887 RTC chips.