mailbox: apcs-ipc: Add support for spss on sdm855

The spss interrupt register does not share the same region as other
edges. This change adds the correct configuration for spss on sdm855.

Change-Id: I9dab4adf2326d686b8744b41a06b4b8686706b22
Signed-off-by: Chris Lew <clew@codeaurora.org>