clk: stm32f4: Switch to clk_divider.determine_rate

clk: stm32f4: Switch to clk_divider.determine_rate

.determine_rate is meant to replace .round_rate in CCF in the future.
Switch over to .determine_rate now that clk_divider_ops has gained
support for that.

Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20210702225145.2643303-5-martin.blumenstingl@googlemail.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>