ARM: Exynos: migrate DCSCB to the new MCPM backend abstraction

The custom suspend callback is removed for this change. The extra call
to exynos_cpu_power_up(() that was present at the end of exynos_suspend()
is now relocated to the cpu_is_up callback.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>