malta: correct UART baudrate

CONFIG_SYS_NS16550_CLK specifies the rate of the clock 16x the baud
rate. The SMSC FDC37M81x datasheet states that a divider of 1 results in
a UART at 115200 baud, thus the x16 clock rate is 115200 * 16.
Previously the divider was left at 0 which led to a rate of 38400 baud
regardless of CONFIG_BAUDRATE or the baudrate environment variable.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>