ARM: dts: r8a7794: Add SYSC PM Domains

Add a device node for the System Controller.
Hook up the Cortex-A7 CPU cores and the Cortex-A7 L2 cache/SCU to their
respective PM Domains.

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>