sched/walt: Avoid walt irq work in offlined cpu

Avoid walt irq work in offlined cpu.

Change-Id: Ia4410562f66bfa57daa15d8c0a785a2c7a95f2a0
Signed-off-by: Maria Yu <aiquny@codeaurora.org>
[clingutla@codeaurora.org: Resolved trivial merge conflicts]
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
[satyap@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>