rpi3: Detect board revision

Implement VideoCore mailbox interface driver and use it to get the board
revision identifier.

For now it is only used to print the model for debug purposes.

This wiki contains the documentation of the mailbox interface:

https://github.com/raspberrypi/firmware/wiki

Change-Id: I11943b99b52cc1409f4a195ebe58eb44ae5b1d6c
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/plat/rpi3/rpi3_private.h b/plat/rpi3/rpi3_private.h
index a9fbfe4..9d1744e 100644
--- a/plat/rpi3/rpi3_private.h
+++ b/plat/rpi3/rpi3_private.h
@@ -33,4 +33,7 @@
 /* IO storage utility functions */
 void plat_rpi3_io_setup(void);
 
+/* VideoCore firmware commands */
+int rpi3_vc_hardware_get_board_revision(uint32_t *revision);
+
 #endif /*__RPI3_PRIVATE_H__ */