arm: tegra: fix PTM dead locking on tight loop

arm: tegra: fix PTM dead locking on tight loop

If one core is executing a tight loop and generating huge number of PTM
packets on funnel, it will cause other core dead locking on the PTM
flushing during the WFI entrance.

This patch will replace the "cpu_relax()" with "smp_mb()", and it can
relax the CPU tight loop and allow other CPU's PTM packet being flushed
properly.

bug 9622188

Change-Id: Ibe17fc91a74a5446d560f71ef9308f3dbd32f6c9
Signed-off-by: Xin Xie <xxie@nvidia.com>
Reviewed-on: http://git-master/r/419511
Reviewed-by: Automatic_Commit_Validation_User
Tested-by: Mitch Luban <mluban@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Mitch Luban <mluban@nvidia.com>