PM / devfreq: cache_hwmon: Use array for reporting monitor stats

Using an array to report monitor stats instead of hard coded variable
names would allow for cleaner implementations of some cache hwmon
device drivers.

Change-Id: I787bdc12f10a0c8ff3c4195ce229a2987acdfce7
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>