mailbox: prefix new constants with MBOX_

mailbox: prefix new constants with MBOX_

Commit 89e5d7d61600 ("mailbox: remove superfluous internal header")
moved some constants to a public header but forgot to add a mailbox
specific prefix. Add this now to prevent future collisions on a too
generic naming.

Link: https://sashiko.dev/#/patchset/20260327151112.5202-2-wsa%2Brenesas%40sang-engineering.com
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Sudeep Holla <sudeep.holla@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>