[CORE] Add a clock specific API

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
[ARM] Exynos4: Add CMU API support.

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
[ARM] Exynos4: Add clock tree support

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>