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>

arch/arm/mach-integrator/Kconfig

@@ -20,7 +20,7 @@ if ARCH_INTEGRATOR
 
 config ARCH_INTEGRATOR_AP
 	bool "Support Integrator/AP and Integrator/PP2 platforms"
-	select CLKSRC_MMIO
+	select INTEGRATOR_AP_TIMER
 	select MIGHT_HAVE_PCI
 	select SERIAL_AMBA_PL010 if TTY
 	select SERIAL_AMBA_PL010_CONSOLE if TTY