driver: serial: uart_stm32: Calculate suitable PRESCALER value

Current driver set a fixed prescaler value for the lpuart
that caused certain baudrate configurations to fail due to
LPUARTDIV overflow the LPUART_BRR register.

This PR attempt to calculate a suitable PRESCALER for the
selected baudrate, throws error and return if it couldn't get
an optimal one.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>