clk: renesas: cpg-mssr: Combine driver-private and clock array allocation

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

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