clk: renesas: rcar-gen3: Improve arithmetic divisions

- Use div64_ul() instead of div_u64() if the divisor is unsigned long,
    to avoid truncation to 32-bit on 64-bit platforms,
  - Use div_u64() for 64-by-32 divisions.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20190830134515.11925-3-geert+renesas@glider.be