rpi3: Move VC mailbox driver into generic drivers directory

To allow sharing the driver between the RPi3 and RPi4, move the mailbox
driver into the generic driver directory.

Change-Id: I463e49acf82b02bf004f3d56482b7791f3020bc0
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/rpi/rpi3/platform.mk b/plat/rpi/rpi3/platform.mk
index c011c0a..b0e7b70 100644
--- a/plat/rpi/rpi3/platform.mk
+++ b/plat/rpi/rpi3/platform.mk
@@ -21,7 +21,8 @@
 				plat/rpi/rpi3/aarch64/plat_helpers.S	\
 				plat/rpi/rpi3/rpi3_bl1_setup.c		\
 				plat/rpi/rpi3/rpi3_io_storage.c		\
-				plat/rpi/rpi3/rpi3_mbox.c
+				drivers/rpi3/mailbox/rpi3_mbox.c	\
+				plat/rpi/rpi3/rpi_mbox_board.c
 
 BL2_SOURCES		+=	common/desc_image_load.c		\
 				drivers/io/io_fip.c			\