MIPS: mscc: Add luton PC0B91 device tree

Add a device tree for the Microsemi Luton PCB091 evaluation board.

It is based on the work of Lars Povlsen <lars.povlsen@microchip.com>.

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>