serial: stm32: implement prescaler tuning to compute low baudrate

serial: stm32: implement prescaler tuning to compute low baudrate

In the case of high USART input clock and low baud rate, BRR value
is not enough to get correct baud rate. So here we use USART prescaler to
divide USART input clock to get the correct baud rate.

PRESC register is only available since stm32h7.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20240112095300.2004878-2-valentin.caron@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>