clk: renesas: mstp: Combine group-private and clock array allocation

Make mstp_clock_group.clks[] a flexible array member, and use the new
struct_size() helper, to combine the allocation of the group-private
structure and array of module clocks.

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