tools/kwbimage: add BAUDRATE option

Offset 0x18 in some Marvell datasheets this field is redacted as
"reserved". This offset is actually a set of options and bits 2:0 allow
the selection of the UART baudrate.

Allow a BAUDRATE option to set the UART baudrate for any messages coming
from the BootROM firmware.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>