We assumed a 16MHz reference frequency for the UART. While this
is true for i.MX1 most of the time it is not true for MX27/MX31.
Also, add handling for the ONEMS register which is present on
newer versions of the chip and pass a sane minimum baudrate to
uart_get_baud_rate().

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>