ARM: mvebu: Instantiate system controller in kirkwood.dtsi

Make use of the mvebu system controller, by placing a node into the
dtsi file.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>