drivers: qcom: rpmh: Cache write_passthru sleep/wake requests

Cache write passthru request for sleep and wake state for the
controller. Flush them first before flushing all other sleep and wake
state requests. This should optimize the number or calls made to the TCS
mailbox controller.

Change-Id: I109a284a5611834361d8970eaa10c91bdcf8207e
Signed-off-by: Lina Iyer <ilina@codeaurora.org>