intel: Refactor common platform code [3/5]

Pull out mailbox driver into common area as they can be shared between
intel's socfpga platform

Signed-off-by: Hadi Asyrafi <muhammad.hadi.asyrafi.abdul.halim@intel.com>
Change-Id: I4064de1ec668931d77abcb7804f6952b70d33716
diff --git a/plat/intel/soc/agilex/socfpga_sip_svc.c b/plat/intel/soc/agilex/socfpga_sip_svc.c
index 6a1c957..16e3c03 100644
--- a/plat/intel/soc/agilex/socfpga_sip_svc.c
+++ b/plat/intel/soc/agilex/socfpga_sip_svc.c
@@ -9,7 +9,7 @@
 #include <common/runtime_svc.h>
 #include <tools_share/uuid.h>
 
-#include "agilex_mailbox.h"
+#include "socfpga_mailbox.h"
 
 /* Number of SiP Calls implemented */
 #define SIP_NUM_CALLS		0x3