drivers: soc: qcom: use hw counter values for PMU snapshot to RIMPS

RIMPS memlat driver reads the current PMU counter value
using perf_event_read_local() API at the time of core
going offline or entering LPM and saves the current
snapshot to a shared memory for access by RIMPS.

RIMPS reads these values from the shared memory when
core is offline or in LPM state, and reads directly
from PMU hardware when core is online. The value read
from hardware and the one read from shared memory are
going out of sync because perf event driver limits the
counting range of PMU counter to half of the total range
supported by hardware, and it configures the counter to
start counting from midpoint at the time of counter reset
and upon overflow. It keeps adding the delta read from
hardware counters to software maintained counters and it
returns the same through perf_event_read API.
As this is not the exact same value as what is read from
hardware counters, the values read by RIMPS from shared
memory are going out of sync.

To address this get the current snapshot of hardware counter
value from prev_count member of hw_perf_event structure which
is part of perf_event structure, and store that to shared memory.

Change-Id: I71f416270a02f7925b16e576f637cf3579fec6ce
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>