dts: generate multiple device tree blobs

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 <yamada.m@jp.panasonic.com>