clk: renesas: rcar-gen2: Change multipliers and dividers to u8

All multipliers and dividers are small.
Storing them in u8 instead of unsigned int reduces kernel size for a
generic kernel by ca. 0.5 KiB.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20191206133254.23800-1-geert+renesas@glider.be