mips: mscc: build FIT image for Ocelot

Ocelot now has a u-boot port, allow building FIT images instead of relying
on the legacy detection and builtin DTB.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19632/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org