ARM: dts: r8a7791: Add SYSC PM Domains

Add a device node for the System Controller.
Hook up the Cortex-A15 CPU cores and the Cortex-A15 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>