clk: renesas: rcar-gen3: Add support for SCCG/Clean peripheral clocks

On R-Car Gen3 SoCs with a Spread Spectrum Clock Generator (e.g. R-Car
D3), a peripheral clock divider has been added, to select between clean
and spread spectrum parents.

Add a new clock type to the R-Car Gen3 driver core to handle this.
To avoid increasing the size of struct cpg_core_clk, both parents and
dividers are stored in the existing parent resp. div fields.

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