clocksource/drivers/integrator-ap: Add the COMPILE_TEST option

clocksource/drivers/integrator-ap: Add the COMPILE_TEST option

Change the Kconfig option logic to fullfil with the current approach.

A new Kconfig option is added, CONFIG_INTEGRATOR_AP_TIMER and is selected
by the platform. Then the clocksource's Kconfig is changed to make this
option selectable by the user if the COMPILE_TEST option is set. Otherwise,
it is up to the platform's Kconfig to select the timer.

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