clk: renesas: rcar-gen3: Add custom clock for PLLs

Currently the PLLs are modeled as fixed factor clocks, based on initial
settings.  However, enabling CPU boost clock rates requires increasing
the PLL clock rates.

Add a custom clock driver to model the PLL clocks.  This will allow the
Z (CPU) clock driver to request changing the PLL clock rate.

Based on a patch in the BSP by Takeshi Kihara
<takeshi.kihara.df@renesas.com>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20210326120100.1577596-7-geert+renesas@glider.be