TQM5200: fix spurious characters on second serial interface

With this patch PSC3 is configured as UART. This is done, because if
the pins of PSC3 are not configured at all (-> all pins are GPI),
due to crosstalk, spurious characters may be send over the RX232_2_TXD
signal line.

Signed-off-by: Martin Krause <martin.krause@tqs.de>