ARM: dts: r8a7779: Add SYSC PM Domains

Add a device node for the System Controller.
Hook up ARM CPU cores 1-3 to their respective PM Domains.
Note that ARM CPU core 0 cannot be shut off.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>