clk: renesas: rcar-gen3: Parameterise Z and Z2 clock fixed divisor

Parameterise Z and Z2 clock fixed divisor to allow clocks with a fixed
divisor other than 2, the value used by all such clocks supported to date.

This is in preparation for supporting the Z2 clock on the R-Car E3
(r8a77990) SoC which has a fixed divisor of 4.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[simon: squashed several patches; rewrote changelog; added r8a774a1 change]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>