arm: bcm7xxx: Add a devicetree file

Add an empty devicetree file for these boards. It seems to be possible to
obtain a real one from another bootloader called 'bolt' but I will leave
this to the maintainer.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index b07a1b3..8bb15b9 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -1092,6 +1092,8 @@
 
 dtb-$(CONFIG_TARGET_BCMNS3) += ns3-board.dtb
 
+dtb-$(CONFIG_ARCH_BCMSTB) += bcm7xxx.dtb
+
 dtb-$(CONFIG_ASPEED_AST2500) += ast2500-evb.dtb
 dtb-$(CONFIG_ASPEED_AST2600) += ast2600-evb.dtb