dt-bindings: clock: add R8A77980 CPG core clock definitions

Add macros usable by the device tree sources to reference the R8A77980
CPG core clocks by index. The data come from the table 8.2e of the R-Car
Series, 3rd Generation User's Manual: Hardware (Rev. 0.80, Oct, 2017),
however I had to add the Z2 clock which is somehow present only on the
figure 8.1e...

Based on the original (and large) patch by Vladimir Barinov.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>