clk: renesas: rz: clk-rz is meant for RZ/A1

The RZ family of Renesas SoCs has several different subfamilies (RZ/A,
RZ/G, RZ/N, and RZ/T).  Clarify that the renesas,rz-cpg-clocks DT
bindings and clk-rz driver apply to RZ/A1 only.

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