omap: Update omap2plus_defconfig to use ttyO instead ttyS

With the omap-serial the device has changed from ttyS to ttyO as
the system may have both omap-serial and 8250 ports.

Note that systems using omap-serial need to be updated to use ttyO[012]
instead of ttyS[012] in the bootloader, CONFIG_CMDLINE, /etc/inittab,
and the root file system with mknod.

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