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>
diff --git a/drivers/clocksource/Makefile b/drivers/clocksource/Makefile
index d888c98..a818431 100644
--- a/drivers/clocksource/Makefile
+++ b/drivers/clocksource/Makefile
@@ -56,7 +56,7 @@
 obj-$(CONFIG_ARM_TIMER_SP804)		+= timer-sp804.o
 obj-$(CONFIG_CLKSRC_METAG_GENERIC)	+= metag_generic.o
 obj-$(CONFIG_ARCH_HAS_TICK_BROADCAST)	+= dummy_timer.o
-obj-$(CONFIG_ARCH_KEYSTONE)		+= timer-keystone.o
+obj-$(CONFIG_KEYSTONE_TIMER)		+= timer-keystone.o
 obj-$(CONFIG_ARCH_INTEGRATOR_AP)	+= timer-integrator-ap.o
 obj-$(CONFIG_CLKSRC_VERSATILE)		+= versatile.o
 obj-$(CONFIG_CLKSRC_MIPS_GIC)		+= mips-gic-timer.o