watchdog: Add support for cpu isolation

Open up interface to allow external subsystem to enable and disable hard
lockup detector.

Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>
[markivx: Forward port to 4.14 and moved sched.h diff to nmi.h]
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>

Change-Id: I42b82e7c7869be1df0837c86d3d97aafb4b064bb
[satyap@codeaurora.org: minor indentation fixes]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>