clk: renesas: Stop enabling legacy DT clock support by default

Since v4.15-rc1, the DTS files for all R-Car Gen2 SoCs have been
converted to the new CPG/MSSR bindings.  Hence it is now safe to no
longer enable legacy DT clock support by default.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>