ppc4xx: Enable auto RS485 mode on PLU405 boards

This patch turns on the auto RS485 mode in the 2nd external
uart on PLU405 boards. This is a special mode of the used
Exar XR16C2850 uart. Because these boards only have a 485 physical
layer connected it's a good idea to turn it on by default.

Signed-off-by: Matthias Fuchs <mf@esd.eu>
Signed-off-by: Stefan Roese <sr@denx.de>