drivers: qcom: rpmh: export controller idle status

Allow the controller state be read by platform drivers. This is useful
for PM drivers which can choose to disallow idle modes when the
controller is busy.

Change-Id: Ic5790c01e9438440ba80be6572fb919501fafc6c
Signed-off-by: Lina Iyer <ilina@codeaurora.org>
Signed-off-by: Raju P.L.S.S.S.N <rplsssn@codeaurora.org>