drivers: serial: ra: reduce uart baud rate error

Using the 8 base clock cycles per bit period setting (instead of 16)
reduces the uart baud rate error when using a 12MHz crystal (found on
many RA Microcontroller development kits boards). This setting also
slightly reduces the error when using the internal 48MHz oscillator,
used by the Arduino UNO R4 Minima board currently support by Zephyr.

Signed-off-by: Ian Morris <ian.d.morris@outlook.com>