ARM: i.MX53: UARTn_TXD pin should not change RXD input select

If both the RXD pad and TXD pad definitions setup
IOMUXC_UARTn_IPP_UART_RXD_MUX_SELECT_INPUT, then
the order of setup will matter. We don't want that.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>