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/platform.mk b/plat/intel/soc/agilex/platform.mk
index b523d9c..bf8fc36 100644
--- a/plat/intel/soc/agilex/platform.mk
+++ b/plat/intel/soc/agilex/platform.mk
@@ -47,7 +47,7 @@
 		plat/intel/soc/common/socfpga_delay_timer.c		\
 		plat/intel/soc/common/socfpga_image_load.c		\
 		plat/intel/soc/agilex/soc/agilex_system_manager.c	\
-		plat/intel/soc/agilex/soc/agilex_mailbox.c		\
+		plat/intel/soc/common/soc/socfpga_mailbox.c		\
 		plat/intel/soc/common/drivers/qspi/cadence_qspi.c	\
 		plat/intel/soc/common/drivers/wdt/watchdog.c		\
 		plat/intel/soc/common/drivers/ccu/ncore_ccu.c
@@ -65,7 +65,7 @@
 		plat/intel/soc/agilex/soc/agilex_reset_manager.c	\
 		plat/intel/soc/agilex/soc/agilex_pinmux.c		\
 		plat/intel/soc/agilex/soc/agilex_clock_manager.c	\
-		plat/intel/soc/agilex/soc/agilex_mailbox.c
+		plat/intel/soc/common/soc/socfpga_mailbox.c
 
 PROGRAMMABLE_RESET_ADDRESS	:= 0
 BL2_AT_EL3			:= 1