arm: Rename hw/arm/arm.h to hw/arm/boot.h

The header file hw/arm/arm.h now includes only declarations
relating to hw/arm/boot.c functionality. Rename it accordingly,
and adjust its header comment.

The bulk of this commit was created via
 perl -pi -e 's|hw/arm/arm.h|hw/arm/boot.h|' hw/arm/*.c include/hw/arm/*.h

In a few cases we can just delete the #include:
hw/arm/msf2-soc.c, include/hw/arm/aspeed_soc.h and
include/hw/arm/bcm2836.h did not require it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190516163857.6430-4-peter.maydell@linaro.org
diff --git a/hw/arm/realview.c b/hw/arm/realview.c
index 05a244d..d42a76e 100644
--- a/hw/arm/realview.c
+++ b/hw/arm/realview.c
@@ -12,7 +12,7 @@
 #include "qemu-common.h"
 #include "cpu.h"
 #include "hw/sysbus.h"
-#include "hw/arm/arm.h"
+#include "hw/arm/boot.h"
 #include "hw/arm/primecell.h"
 #include "hw/net/lan9118.h"
 #include "hw/net/smc91c111.h"