clk: renesas: cpg-mssr: Document suitability for RZ/G1

The Renesas CPG/MSSR driver is already in active use for RZ/G1 since
commits c0b2d75d ("clk: renesas: cpg-mssr: Add R8A7743 support")
and 9127d54b ("clk: renesas: cpg-mssr: Add R8A7745 support").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>