clocksource/drivers/tango-xtal: Replace code by clocksource_mmio_init

The current code to initialize, register and read the clocksource is
already factored out in mmio.c via the clocksource_mmio_init function.

Factor out the code with the clocksource_mmio_init function.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index db240cb..b251013 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig
@@ -351,6 +351,7 @@
 	bool "Clocksource for Tango SoC" if COMPILE_TEST
 	depends on ARM
 	select CLKSRC_OF
+	select CLKSRC_MMIO
 	help
 	  This enables the clocksource for Tango SoC