serial: mxc: enable the RX pipeline

on imx8(mm) the RXDMUXSEL needs to be set for data going over the wire
(as observable on a connected 'scope) to actually make it into the
RXFIFO

the reference manual is not overly clear about this, and only
mentiones that "UCR3_RXDMUXSEL should always be set." - and since the
CR3 register reverts to its reset values after setting the baudrate,
setting this bit is done during '_mxc_serial_setbgr'

Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>