soc: apple: rtkit: Port to the internal mailbox driver

Now that we have a mailbox driver in drivers/soc/apple, port the RTKit
code to it. This mostly just entails replacing calls through the mailbox
subsystem with direct calls into the driver.

Acked-by: Eric Curtin <ecurtin@redhat.com>
Acked-by: Neal Gompa <neal@gompa.dev>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Hector Martin <marcan@marcan.st>