arm/dt: Add dtb make rule

Add a make rule to compile dt blobs for ARM.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Tested-by: Jason Liu <jason.hui@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>