clk: renesas: div6: Implement range checking

Consider the minimum and maximum clock rates imposed by clock users when
calculating the most appropriate clock rate in the .determine_rate()
callback.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/35ceb262c71f1b2e9864a39bde9dafd78b2981f4.1617281699.git.geert+renesas@glider.be