Add macros usable by the device tree sources to reference the R8A77970
CPG core clocks by index. The data come from the table 8.2c of R-Car
Series, 3rd Generation User's Manual: Hardware (Rev. 0.55, Jun. 30, 2017).

Based on the original (and large) patch by Daisuke Matsushita
<daisuke.matsushita.ns@hitachi.com>.
Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>