arm: Always keep the dtb section on objcopy

The dtb blob section must always be present in the resulting image.
Either if OF_EMBEDED is used or if unit tests include dtb blobs.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>