arm: rpi: Add a file to handle messages

The bcm283x chips provide a way for the ARM core to communicate with the
graphics processor, which is in charge of many things. This is handled by
way of a message prototcol.

At present the code for sending message (and receiving a reply) is spread
around U-Boot, primarily in the board file. This means that sending a
message from a driver requires duplicating the code.

Create a new message implementation with a function to support powering on
a subsystem as a starting point.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-bcm283x/Makefile b/arch/arm/mach-bcm283x/Makefile
index 5cb1b2f..b5f606e 100644
--- a/arch/arm/mach-bcm283x/Makefile
+++ b/arch/arm/mach-bcm283x/Makefile
@@ -5,4 +5,4 @@
 #
 
 obj-$(CONFIG_BCM2835) += lowlevel_init.o
-obj-y	+= init.o reset.o mbox.o phys2bus.o
+obj-y	+= init.o reset.o mbox.o msg.o phys2bus.o