clocksource/drivers/bcm2835: Add the COMPILE_TEST option

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

A new Kconfig option is added, CONFIG_BCM2835_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 bc66981..008d8a0 100644
--- a/drivers/clocksource/Makefile
+++ b/drivers/clocksource/Makefile
@@ -19,7 +19,7 @@
 obj-$(CONFIG_CLKSRC_DBX500_PRCMU)	+= clksrc-dbx500-prcmu.o
 obj-$(CONFIG_ARMADA_370_XP_TIMER)	+= time-armada-370-xp.o
 obj-$(CONFIG_ORION_TIMER)	+= time-orion.o
-obj-$(CONFIG_ARCH_BCM2835)	+= bcm2835_timer.o
+obj-$(CONFIG_BCM2835_TIMER)	+= bcm2835_timer.o
 obj-$(CONFIG_ARCH_CLPS711X)	+= clps711x-timer.o
 obj-$(CONFIG_ARCH_ATLAS7)	+= timer-atlas7.o
 obj-$(CONFIG_ARCH_MOXART)	+= moxart_timer.o