It is convenient to have all device trees on the same SoC compiled.
It allows for later easy repackaging without the need to re-run
the make file.

- Build device trees with the same SoC under arch/$(ARCH)/dts

- Copy the one specified by CONFIG_DEFAULT_DEVICE_TREE or
DEVICE_TREE=... to dts/dt.dtb

Signed-off-by: Masahiro Yamada