c6x: enable building all dtbs

Enable the 'dtbs' target for c6x. This allows building all the dts
files in arch/c6x/boot/dts/ for enabled platforms or when
COMPILE_TEST and OF_ALL_DTBS are enabled.

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>