drivers: mailbox: query controller idle status

Power drivers that are interested in knowing the current state of the
controller so they can turn down idle states may do so using the
->is_idle() callback on the controller. The call will default to false,
if the controller does not support the ->is_idle callback.

Change-Id: Idf4290d3a651604c8637425abfa25cedc9a02d10
Signed-off-by: Lina Iyer <ilina@codeaurora.org>