sandbox: Build a device tree file for sandbox

Add support for building a device tree for sandbox's CONFIG_OF_HOSTFILE
option to make it easier to use device tree with sandbox.

This adjusts the Makefile to build a u-boot.dtb file which can be passed
to sandbox U-Boot with:

   ./u-boot -d u-boot.dtb

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/Makefile b/arch/sandbox/dts/Makefile
new file mode 100644
index 0000000..a4c980b
--- /dev/null
+++ b/arch/sandbox/dts/Makefile
@@ -0,0 +1,11 @@
+dtb-$(CONFIG_SANDBOX) += sandbox.dtb
+
+targets += $(dtb-y)
+
+DTC_FLAGS += -R 4 -p 0x1000
+
+PHONY += dtbs
+dtbs: $(addprefix $(obj)/, $(dtb-y))
+	@:
+
+clean-files := *.dtb
diff --git a/arch/sandbox/dts/sandbox.dts b/arch/sandbox/dts/sandbox.dts
new file mode 100644
index 0000000..96a4438
--- /dev/null
+++ b/arch/sandbox/dts/sandbox.dts
@@ -0,0 +1,20 @@
+/dts-v1/;
+
+/ {
+	triangle {
+		compatible = "demo-shape";
+		colour = "cyan";
+		sides = <3>;
+		character = <83>;
+	};
+	square {
+		compatible = "demo-shape";
+		colour = "blue";
+		sides = <4>;
+	};
+	hexagon {
+		compatible = "demo-simple";
+		colour = "white";
+		sides = <6>;
+	};
+};