soc: apple: mailbox: Rename config symbol to APPLE_MAILBOX

With the original owner of APPLE_MAILBOX removed, let's rename the new
APPLE_MBOX to the old name. This avoids .config churn for downstream
users, and leaves us with an identical config symbol and module name as
before.

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>