clk: renesas: rcar-gen3: Add support for ZG clock

clk: renesas: rcar-gen3: Add support for ZG clock

A clock used for the 3D graphics appears to be common
among multiple SoC's, so add a generic gen3 clock
for clocking the graphics.  This is similar to the
cpg_z_clk, with a different frequency control register
and different flags.  Instead of duplicating the code,
make cpg_z_clk_register into a helper function and
call the help function with the FCR and flags as
a parameter.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230617150302.38477-1-aford173@gmail.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>