soc: apple: rtkit: Switch to atomic mailbox API

soc: apple: rtkit: Switch to atomic mailbox API

Turns off tx_block and use the tx_done callback to implement our own
completion handling. This makes our usage of the mailbox API
atomic-safe, and using a per-message completion is also more robust.

Then, implement apple_rtkit_send_message_atomic which polls for
transmission to be complete instead of using a completion. This is
required to send RTKit messages from atomic contexts, notably the
final system shutdown message to SMC.

Signed-off-by: Hector Martin <marcan@marcan.st>