clocksource/drivers/armv7m_systick: Add the COMPILE_TEST option

In order to increase the compilation test coverage, add the COMPILE_TEST
so the driver can be compiled even if it does not belong to the platform
or the architecture.

The io.h header inclusion is also added as it the driver does not compile
on UM platform.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>