clocksource/drivers/keystone: Add the COMPILE_TEST option

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

A new Kconfig option is added, CONFIG_KEYSTONE_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-keystone/Kconfig

@@ -4,7 +4,7 @@ config ARCH_KEYSTONE
 	depends on ARM_PATCH_PHYS_VIRT
 	select ARM_GIC
 	select HAVE_ARM_ARCH_TIMER
-	select CLKSRC_MMIO
+	select KEYSTONE_TIMER
 	select ARM_ERRATA_798181 if SMP
 	select COMMON_CLK_KEYSTONE
 	select ARCH_SUPPORTS_BIG_ENDIAN