ARM: SAMSUNG: add clock_tree debugfs file in clock

Add a clock/clock_tree debugfs file that shows the entire clock
hierarchy including usage counts and rates.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
[kgene.kim@samsung.com: silence checkpatch complaining]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>