rpi3: Move rpi3_hw.h header file to include/rpi_hw.h

With the advent of Raspberry Pi 4 support, we need to separate some
board specific headers between the RPi3 and RPi4.
Rename and move the "rpi3_hw.h" header, so that .c files just include
rpi_hw.h, and automatically get the correct version.

Change-Id: I03b39063028d2bee1429bffccde71dddfe2dcde8
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/rpi/rpi3/rpi3_mbox.c b/plat/rpi/rpi3/rpi3_mbox.c
index 2db605e..03b395e 100644
--- a/plat/rpi/rpi3/rpi3_mbox.c
+++ b/plat/rpi/rpi3/rpi3_mbox.c
@@ -12,7 +12,7 @@
 #include <common/debug.h>
 #include <lib/mmio.h>
 
-#include "rpi3_hw.h"
+#include <rpi_hw.h>
 
 /* This struct must be aligned to 16 bytes */
 typedef struct __packed __aligned(16) rpi3_mbox_request {