ARM: dts: r8a7792: add power domain index macros

Add macros usable by the device tree sources to reference R8A7792 SYSC power
domains by index.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>