clocksource/drivers/prcmu: Fix Kconfig and add COMPILE_TEST option

Let the platform's Kconfig to select the clock instead of having a reverse
dependency from the driver to the platform options.

Add the COMPILE_TEST option for the compilation test coverage.

This change is debatable as the option itself in the Kconfig allows to
select the driver for the platform or not. This change will make the prcmu
timer always selected.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>