km/uart: port UART interface of KM Kirkwood boards to driver model

Activate the driver model for the serial interface in the KM Kirkwood
Kconfig file. The associated preprocessor definitions could be removed
from the header file. However, the clock of 200 MHz needs to be declared
in the device tree.

Signed-off-by: Pascal Linder <pascal.linder@edu.hefr.ch>
Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
Signed-off-by: Stefan Roese <sr@denx.de>