omap2+: Initialize serial port for dynamic remuxing for n8x0

Use omap_serial_init_port so we can let the serial code handle the
remuxing of the RX pads. Note that this patch alone is not enough
and additional GPIO related patches are needed.

Only initialize uart3_rx_irrx pin, the other uart pins can be
stay static.

Signed-off-by: Tony Lindgren <tony@atomide.com>