soc: apple: rtkit: Add a private pointer to apple_rtkit_shmem

This allows downstream consumers to keep track of private data for shmem
mappings. In particular, the Rust abstraction will use this to safely
drop data associated with a mapping when it is unmapped.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Reviewed-by: Eric Curtin <ecurtin@redhat.com>
Signed-off-by: Hector Martin <marcan@marcan.st>