clk: renesas: r8a779h0: Drop CLK_PLL2_DIV2 to clarify ZCn clocks

Early revisions of the R-Car V4M Series Hardware User’s Manual
contained an incorrect formula for the CPU core clocks:

    ZCnφ = (PLL2VCO x 1/2) x mult/32

Dang-san fixed this by using CLK_PLL2_DIV2 instead of CLK_PLL2 as the
parent clock.

In Rev.0.70 of the documentation, the formula was corrected to:

    ZCnφ = (PLL2VCO x 1/4) x mult/32

As the CPG Block Diagram now shows a separate 1/4 post-divider for PLL2,
the use of CLK_PLL2_DIV2 is a recurring source of confusion.  Hence get
rid of CLK_PLL2_DIV2, and include the proper 1/4 post-divider in the
invocation of the DEF_GEN4_Z() macro, like is done on other R-Car Gen4
(and Gen3) SoCs.

Reported-by: Vinh Nguyen <vinh.nguyen.xz@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/0d2789cac2bf306145fe0bbf269c2da5942bb68f.1728377724.git.geert+renesas@glider.be