MIPS: mscc: build FIT image for Luton

Luton now has already an u-boot port so let's build FIT images.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>