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: NDaniel Lezcano <daniel.lezcano@linaro.org>

Sigma Designs Tango platforms provide a 27 MHz crystal oscillator.
Use it for clocksource, sched_clock, and delay_timer.
Signed-off-by: NMarc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>