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/stratix10/bl2_plat_setup.c b/plat/intel/soc/stratix10/bl2_plat_setup.c
index 1cb9c25..7376b41 100644
--- a/plat/intel/soc/stratix10/bl2_plat_setup.c
+++ b/plat/intel/soc/stratix10/bl2_plat_setup.c
@@ -27,9 +27,9 @@
 #include "s10_reset_manager.h"
 #include "s10_clock_manager.h"
 #include "s10_pinmux.h"
-#include "include/s10_mailbox.h"
 #include "qspi/cadence_qspi.h"
 #include "socfpga_handoff.h"
+#include "socfpga_mailbox.h"
 #include "socfpga_private.h"
 #include "wdt/watchdog.h"