clocksource/drivers/versatile: Add the COMPILE_TEST option

Change the Kconfig option logic to fullfil with the current approach,
allowing the user to compile the driver on different platforms.

The current option let the user to select the clocksource or not.

The Kconfig option policy is to let the platform to select the
timer automatically.

Add the COMPILE_TEST option, so the prompt to select the driver will
be showed only when COMPILE_TEST is set and will let this driver
to compile on different platform, thus increasing the compilation
test coverage.

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