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>