arm: mps2_an385: remove setting of baud rate in defconfig

We now get the baud rate for the serial port from the device tree so we
don't need to be setting it in the defconfig.

The baud rate changed from 38400 to 115200 when we introduced the device
tree for MPS2.  The default baud rate was based on matching mbed, but we
use 115200 to match the standard default we have for most boards now.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>