clocksource: Kconfig: Let EM_TIMER_STI depend on HAS_IOMEM

In 'em_sti.c', it will call devm_ioremap_resource() which need
HAS_IOMEM. So need let EM_TIMER_STI depend on HAS_IOMEM, too.

The related error (with allmodconfig under score):

  LD      init/built-in.o
em_sti.c:(.text.em_sti_probe+0x84): undefined reference to `devm_ioremap_resource'
make: *** [vmlinux] Error 1

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>