lib: utils/mpxy: Remove p2a_db_index from RPMI system MSI attributes

The discovery of P2A doorbell system MSI index is now through RPMI
shared memory DT node so remove p2a_db_index from RPMI system MSI
attributes and access it as a mailbox channel attribute.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20250512083827.804151-5-apatel@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>

include/sbi_utils/mailbox/rpmi_msgprot.h

@@ -285,7 +285,6 @@ enum rpmi_sysmsi_service_id {
 struct rpmi_sysmsi_get_attributes_resp {
 	s32 status;
 	u32 sys_num_msi;
-	u32 p2a_db_index;
 	u32 flag0;
 	u32 flag1;
 };