dt-bindings: clock: Add R8A77970 CPG core clock definitions

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>