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>