clk: renesas: div6: Switch to .determine_rate()

As the .round_rate() callback returns a long clock rate, it cannot
return clock rates that do not fit in signed long, but do fit in
unsigned long.  Hence switch the DIV6 clocks on SH/R-Mobile and R-Car
SoCs from the old .round_rate() callback to the newer .determine_rate()
callback, which does not suffer from this limitation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/7fd8c45cd8bf5c6d928ca69c8b669be35b93de09.1617281699.git.geert+renesas@glider.be