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/bl2_plat_setup.c b/plat/intel/soc/agilex/bl2_plat_setup.c
index d65049d..ece7b9c 100644
--- a/plat/intel/soc/agilex/bl2_plat_setup.c
+++ b/plat/intel/soc/agilex/bl2_plat_setup.c
@@ -17,7 +17,6 @@
 #include <platform_def.h>
 
 #include "agilex_clock_manager.h"
-#include "agilex_mailbox.h"
 #include "agilex_memory_controller.h"
 #include "agilex_pinmux.h"
 #include "agilex_reset_manager.h"
@@ -26,6 +25,7 @@
 #include "ccu/ncore_ccu.h"
 #include "qspi/cadence_qspi.h"
 #include "socfpga_handoff.h"
+#include "socfpga_mailbox.h"
 #include "socfpga_private.h"
 #include "wdt/watchdog.h"