clk: renesas: rcar-gen4: Add support for variable fractional PLLs

The custom clock driver that models PLL clocks on R-Car Gen4 supports
PLL2 on R-Car V4H/V4M only, while PLL3, PLL4, and PLL6 use the same
control register layout.

Extend the existing support to PLL3, PLL4, and PLL6, and introduce a new
clock type and helper macro to describe these PLLs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/84ead759782560ec5643711e6bdd787a751053ce.1721648548.git.geert+renesas@glider.be