serial/esp32_uart: use prescaler when available

esp32s3 variant of the esp32 UART has limited baudrate divisor range
that does not allow it to use 9600 and lower rates with 40MHz input
clock. Use clock prescaler present in this UART variant to help with
that.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Link: https://lore.kernel.org/r/20231018191252.1551972-1-jcmvbkbc@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>