dt-bindings: clk: Add r8a774b1 CPG Core Clock Definitions

Add all RZ/G2N Clock Pulse Generator Core Clock Outputs, as listed in
Table 8.2d ("List of Clocks [RZ/G2N]") of the RZ/G2N Hardware User's
Manual.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Link: https://lore.kernel.org/r/1567666360-28035-1-git-send-email-biju.das@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>