drivers: soc: qcom: rpm_stats: Add mutex lock for shared data

The buffer allocated in file open operations need to be
protected as there can be a possiblity of use-after-free
scenario.

Process A              B
        |              |
      open             |
        |              |
      read started     |
        |             close

Add mutex lock to protect the buffer to avoid this.

"msm_rpmstats_copy_stats" accesses the variable "pdata->read_idx"
without locking. The userspace can invoke the "read" call from
multiple threads which will call "msm_rpmstats_file_read" which
in turn calls "msm_rpmstats_copy_stats".

This can allow the statement "pdata->read_idx++" increment
"read_idx" beyond the limit ("prvdata->num_records") and call
"msm_rpmstats_read_register" with this value.

Also allow reading RPM stats information using sysfs nodes.

The stats are available at
	/sys/power/system_sleep/stats

Change-Id: I031f02bb2694a97ced86da0a9f54d0e434e4ad6d
Signed-off-by: Naresh Malladi <namall@codeaurora.org>