mx31moboard: serial port fix

We get rid of CTS/RTS lines on uart0 on our platform.
This is the port we use as main kernel console. We do not
want it to be blocking because of CTS/RTS signals, not allowing
the system to boot or print messages.

However we often use it with a bluetooth module needing CTS/RTS
lines as backup login in case of trouble. To be able to use it,
we assert CTS low so that the module can always send chars.

Signed-off-by: Valentin Longchamp <valentin.longchamp@epfl.ch>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>