clk: renesas: div6: Combine clock-private and parent array allocation

clk: renesas: div6: Combine clock-private and parent array allocation

Make div6_clock.parents[] a flexible array member, and use the new
struct_size() helper, to combine the allocation of the clock-private
structure and array of parent clocks.

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