clocksource/drivers/mediatek: Add the COMPILE_TEST option

Increase the compilation test coverage by adding the COMPILE_TEST option.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/Kconfig

@@ -228,9 +228,11 @@ config SYS_SUPPORTS_SH_CMT
         bool
 
 config MTK_TIMER
+	bool "Mediatek timer driver" if COMPILE_TEST
 	select CLKSRC_OF
 	select CLKSRC_MMIO
-	bool
+	help
+	  Support for Mediatek timer driver.
 
 config SYS_SUPPORTS_SH_MTU2
         bool