clk: renesas: rcar-gen3: Update Z clock rate formula in comments

The fixed divider in the calculation of the Z and Z2 clock rates was
generalized from a hardcoded value of two to a parameterized value, but
the comments were not updated accordingly.

Fixes: 20cc05ba ("clk: renesas: rcar-gen3: Parameterise Z and Z2 clock fixed divisor")
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>
Link: https://lore.kernel.org/r/20210326120100.1577596-2-geert+renesas@glider.be