dt-bindings: clock: renesas: Document RZ/G2L SoC CPG driver

Document the device tree bindings of the Renesas RZ/G2L SoC clock
driver in Documentation/devicetree/bindings/clock/renesas,rzg2l-cpg.yaml.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210609153230.6967-8-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>