clk: renesas: rcar-gen2: Document R8A7792 support

Renesas R8A7792 SoC is a member of the R-Car gen2 family and so has CPG...

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