arm64: Add dtbs target for building all the enabled dtb files

Based on Rob Herring's patches for arch/arm, this patch adds a dtbs
target to arch/arm64/boot/Makefile.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

arch/arm64/boot/dts/.gitignore

@@ -0,0 +1 @@
+*.dtb