c6x: use common built-in dtb support

Using the common build support for built-in dtb files just requires
adding a .dtb.o target to obj-y.

The dtb now needs to be copied when unflattened because an init section
is used now.

Cc: Mark Salter <msalter@redhat.com>
Cc: Aurelien Jacquiot <jacquiot.aurelien@gmail.com>
Cc: linux-c6x-dev@linux-c6x.org
Signed-off-by: Rob Herring <robh@kernel.org>