clocksource/drivers/nspire: Add the COMPILE_TEST option

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

A new Kconfig option is added, CONFIG_NSPIRE_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/arch/arm/mach-nspire/Kconfig b/arch/arm/mach-nspire/Kconfig
index bc41f26..d498530 100644
--- a/arch/arm/mach-nspire/Kconfig
+++ b/arch/arm/mach-nspire/Kconfig
@@ -7,5 +7,6 @@
 	select ARM_AMBA
 	select ARM_VIC
 	select ARM_TIMER_SP804
+	select NSPIRE_TIMER
 	help
 	  This enables support for systems using the TI-NSPIRE CPU