For VCN FW to detect ASIC type, in order to use different mailbox registers.

V2: simplify codes and fix format issue.
Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Acked-by Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>