imx serial: set RXD mux bit on i.MX27 and i.MX31

RX in i.MX27 and i.MX31 UART lines does not work unless the
"RXD  Muxed Input Select" bit is set on i.MX27 and i.MX31 processors.

This patch sets the missing RXD mux bit in the UCR3 register.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>