serial/uuc_uart: Support higher bitrates than 115200 Bit/s

The maximum bitrate supported depends on the clock rate used in BRG.
This is stored in port.uartclk during probe. Respecting the 16x
oversampling higher bitrates can be supported.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>