drivers: qcom: rpmh: Release memory back to pool for sleep/wake requests

Sleep/wake requests are cached locally in RPMH driver and are not sent
to TCS. So there won't be a response callback from the mailbox driver.
Release the memory allocated for an async request. If the request is a
sync variant, signal the completion object, so there is no indefinite
wait for response from mailbox driver.

Change-Id: Ic29227adb7145c610a8d6e98eb09ae47a69f2cf2
Signed-off-by: Lina Iyer <ilina@codeaurora.org>