sched/core: Add API to set cluster d-state

Add new API to the scheduler to allow low power mode driver to inform
the scheduler about the d-state of a cluster. This can be leveraged by
the scheduler to make an informed decision about the cost of placing a task
on a cluster.

Change-Id: If0fe0fdba7acad1c2eb73654ebccfdb421225e62
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>