gpu: nvgpu: sysfs to put gpu into idle

gpu: nvgpu: sysfs to put gpu into idle

- Add a sysfs "force_idle" to forcibly idle the GPU
- read on this sysfs will return the current status

0 : not in idle (running)
1 : in forced idle state

"echo 1 > force_idle" will force the gpu into idle
"echo 0 > force_idle" will cause the gpu to resume

Bug 1376916
Bug 1487804

Change-Id: I48dfd52e0d14561220bc4baea0776d1bdfaa7ea5
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
(cherry picked from commit 44d89a68bf6c9034c8bf9f5111d733290e7cb71e)
Reviewed-on: http://git-master/r/421699
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
GVS: Gerrit_Virtual_Submit