Add macros usable by the device tree sources to reference R8A7745 SYSC power
domains by index.

Based on the original (and large) patch by Dmitry Shifrin
<dmitry.shifrin@cogentembedded.com>.
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>