rtc: at91sam: add 'depends on HAS_IOMEM' to fix unmet dependency

This config select's MFD_SYSCON, but does not depend on HAS_IOMEM.

Compile testing on architecture without HAS_IOMEM causes "unmet
direct dependencies" in Kconfig phase.

Detected by "make ARCH=score allyesconfig".
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/Kconfig

@@ -1423,6 +1423,7 @@ config RTC_DRV_AT91RM9200
 config RTC_DRV_AT91SAM9
 	tristate "AT91SAM9 RTT as RTC"
 	depends on ARCH_AT91 || COMPILE_TEST
+	depends on HAS_IOMEM
 	select MFD_SYSCON
 	help
 	  Some AT91SAM9 SoCs provide an RTT (Real Time Timer) block which