clk: renesas: Add r8a7745 CPG Core Clock Definitions

Add macros usable by the device tree sources to reference the R8A7745
CPG clocks by index. The data comes from Table 7.2c in revision 1.00 of
the RZ/G Series User's Manual.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>