clocksource/atmel-st: Add 'depends on HAS_IOMEM' to fix unmet dependency

The ATMEL_ST config selects 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".

Add the proper dependency to the ATMEL_ST config.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/1520335233-11277-1-git-send-email-yamada.masahiro@socionext.com