PM / devfreq: Add clearing transitions stats

Add clearing transition table and time in states devfreq statistics
by writing 0 (zero) to trans_stat file in devfreq sysfs. An example use
is like following:

echo 0 > /sys/class/devfreq/devfreqX/trans_stat

Signed-off-by: Kamil Konieczny <k.konieczny@samsung.com>
[cw00.choi: Edit return value if entering the wrong value for reset
 and use arrary3_size() to get the size of 3-dimensional array]
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>