PM / devfreq: memlat: Move per-CPU stats inside device nodes

Currently each memlat monitor stores details such as previous sample
timestamp and perf counts in per-CPU structures. This can result in
multiple monitors overwriting each others' data.
Move these stats within per device structures so that each of the
monitors have exclusive access to them.

Change-Id: I0a60e95eebd72f0e602cbf9cba05380324b75b3f
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>