soc: apple: rtkit: Fix buffer address field width

The buffer address field is missing two bits. This matters for the GPU,
which uses upper-half 64-bit addresses on the ASC and those get sign
extended from the mailbox message field, so the right number of high
bits need to be set.

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