arm64: dts: r8a7795: Add SYSC PM Domains

Add a device node for the System Controller.
Hook up the Cortex-A57 CPU cores and the Cortex-A57 and Cortex A53 L2
caches/SCUs 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>