dt-bindings: can: rcar_can: Complete documentation for RZ/G2[EM]

Add missing RZ/G2[EM] CANFD clock specific documentation.

Fixes: 868b7c0f ("dt-bindings: can: rcar_can: Add r8a774a1 support")
Fixes: d703a52e ("dt-bindings: can: rcar_can: Add r8a774c0 support")
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Chris Paterson <Chris.Paterson2@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>