ARM: kirkwood: Introduce MBus DT node

Add a minimal MBus node, just to allow the MBus driver to probe.
Follow-up patches will migrate the rest of the nodes appropriately.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>