mm: mm_event: make capture period configurable

mm: mm_event: make capture period configurable

This patch makes per-process mm event capture inteval configurable.
Default is 500ms but admin can change it by below knob.

	/sys/kernel/debug/mm_event/period_ms

The unit is millisecond.

Mot-CRs-fixed: (CR)

Bug: 80168800
Change-Id: I3b2de3dd5c4a519a2e5e20f1ef0d5f9a4c7afc8a
Signed-off-by: Minchan Kim <minchan@google.com>
Reviewed-on: https://gerrit.mot.com/1453719
SLTApproved: Slta Waiver
SME-Granted: SME Approvals Granted
Tested-by: Jira Key
Reviewed-by: Xiangpo Zhao <zhaoxp3@motorola.com>
Submit-Approved: Jira Key