clk: renesas: rcar-gen3: Parameterise Z and Z2 clock offset

Parameterise the offset of control bits within the FRQCRC register
for Z and Z2 clocks.

This is in preparation for supporting the Z2 clock on the R-Car E3
(r8a77990) SoC which uses a different offset for control bits to
other, already, supported SoCs.

As suggested by Geert Uytterhoeven.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>